"A couple hundred dollars" isn't going to go very far, I"m afraid.
1. LENS.
The first thing I'd suggest you consider, especially for landscape photography, is a wider angle lens... But most cost $500 or more. Among those, the Canon EF-S 10-22mm USM is one of the very best... but it's also more than you say you want to spend.
A new and lower priced alternative is the new EF-S 10-18mm IS STM Canon has just announced. I haven't used it (few people have) because it's not in stock anywhere yet... and may be a bit hard to find for a little while. This lens appears to be reasonably compact (uses 67mm filters) and is going to be selling for about $300. The separately sold lens hood (EW-73C) also isn't available quite yet, but appears to be priced under $30. Not sure I need IS on a lens with such short focal lengths... but, hey, it never hurts to have it! STM focus drive should be good, too. Now, none of us have had a chance to use this lens yet, but taking Canon at their word with what they say about it, should be quite nice.
Some other things that may be within your budget of "a couple hundred dollars"....
2. TRIPOD
If you don't already have one, I'd suggest a good tripod. But, be careful. Do not go too cheap, too compact or too lightweight with this, or you'll find yourself not using it or replacing it pretty fast. One that seems a really good deal to me is the Adorama Flashpoint F1228 kit for about $220 including shipping. This is a clone of Gitzo "Series 2" Reporter carbon fiber tripod, with a rather standard, but decent looking ballhead with quick release. (I use several actual Gitzo, which are excellent but a whole lot more expensive, but I also use much heavier cameras and lenses than you).
I have also heard some good things about Dolica tripods, but have no real experience with them. They offer several carbon fiber models, which is what I'd recommend. There are some other fairly affordable brands, as well as some that cost a great deal more. Just be careful, check them out in person if at all possible, and try to avoid too light, flimsy and poorly made.
Bought right, a tripod might be a once-in-a-lifetime purchase. A carbon fiber tripod costs a bit more, but is lighter than aluminum tripod of the same capacity rating. Also, carbon fiber absorbs small vibrations, where metal "rings" and reverberates. Plus, CF is a bit nicer to handle in hot and cold weather.
A tripod does a number of things for you and a lot of landscape photographers wouldn't think of shooting without one. Of course a tripod steadies your shot and allows you to make longer exposures than are possible handheld. Another thing a tripod does is force you to slow down and more carefully consider the shot(s) you are going to take. This usually results in much nicer photos. Someone already mentioned doing multi-shot panoramas, and a tripod will nicely facilitate doing that, too.
3. LENS HOODS
If you don't already have them, get the matching lens hoods for your lenses. I believe the 18-55mm uses an EW-63C hood, and the 75-300 uses an ET-60 hood. Just get in the habit of using lens hoods. They protect your lenses from oblique light and physical bumps. I see people shooting without hoods all the time... and that makes no sense at all to me. A lens hood is a simple, quick & easy to use, and inexpensive accessory that can only help your images. I rarely shoot without one.
4. FILTERS
And, if you don't already have some, a filter or two might be really helpful. First and foremost, I'd recommend a high quality Circular Polarizer. I believe your 18-55mm lens uses 58mm filters (double check that) and I'd suggest one to fit that lens first, since it's probably the lens you'll use most for landscape photography. A C-Pol filter will deepen the blue of the sky and make white clouds "pop". It also intensifies colors by cutting reflections from foliage and other surfaces. You can use it to control reflections off water and other surfaces, too. It's one of the most widely useful filters, as well as one of the most impossible to replicate with digital softwares. I recommend B+W MRC, XS-Pro, and Kaesemann; Hoya HD, HD2, HMC an SHMC; and Marumi DHG; among others. You don't want to leave a C-Pol on your lens all the time... it cuts one to two stops of light. But you will find it useful many times, once you learn to use it.
Another type of filter thats popular is a Neutral Density or ND. These are overall gray, come in varying strenght and are used to reduce the amoung of light reaching the sensor... so that slower shutter speeds and larger apertures can be used deliberately. You've probably seen those scenic shots with creamy looking moving water... whether a stream, waterfall or ocean suft. Most of those types of shots require some sort of ND filter, when shot with a digital camera. That's because most digital cameras don't got below about ISO 100 or even 200. A few go to ISO 50. This limits how slow a shutter speed can be used when shooting in daylight. If you get this type filter, you will probably want a couple different strengths... they come in one, two, three, 5, 8, 10 and even more stop strengths. There are "variable" (rotate to change the strength, similar to a C-Pol), but these tend to give uneven effects and sometimes cause unwanted color tints. Note: ND filters are also often needed for videography, where frame rate limitations limit the aperture that can be used in available light.
Another type of filter that's historically been quite popular among landscape photographers is the Graduated Neutral Density or ND Grad. These are half clear, half gray in varying strengths, and are used to balance an overly bright sky with a foreground in an image, for example. With this type filter, don't buy the round, screw in type. You need to be able to position the gradation line differently for each image, so need to use the rectangular type of filter, that fits into a special holder that mounts on the front of the lens, allowing you to rotate and slide the filter as needed.
I have a nice set of ND Grads that date back to when I shot with film, but frankly I hardly use them anymore. Shooting digital, I now can do a better job with post-processing techniques... combining multiple exposures, or multi-processing a single RAW file and then recombining the "correctly exposed" portion from each. This is far more accurate, precise and controllable than was ever possible with the filters.
5. SOFTWARE
BUt that leads to another thing.... If not already doing so, learn to shoot RAW and post-process your images. The Canon supplied softwares that came with your camera are pretty good, but you might want to look at an all-in-one package with some more capabilities, such as Adobe Elements. If so, get a book or two about it, too... to help you get up to speed using it quickly and more easily. RAW image files are like a digital negative, of sorts. It's everything the camera captured at the time of exposure, but nearly always needs some more work to turn it into a usable image (a JPEG, TIFF, PNG, or whatever). In contrast, a JPEG shot in camera throws away a lot of data, and is far less "adjustable" after the fact... so you have to get your exposure settings, white balance, etc. right the first time... there is less opportunity to change or correct it later.
6. CALIBRATION
You also might consider calibrating your computer. There are various devices and softwares available for this purpose, but the general idea is that most computer monitors do not give very accurate color and nearly all are way, way too bright. This makes it hard to accurately work with your images and can drive you up a walll trying to make prints. The secret is calibration. This doesn't have to be terribly expensive... and can pay for itself if you make a lot of prints, in savings of paper and ink, or reprints if you send out your printing.
7. BOOKS & CLASSES
In general, the $18 book "Understanding Exposure" by Brian Peterson might be the best money you ever spend on your photography. I highly recommend it, no matter your level of experience but especially for novices. Buy it. Read it. Study it. Learn it. Your photography will be much better and you'll get more out of your gear.
There are other good books you might look at. Some are specific for your particular camera, field guides that will help you get the best out of it. Others are specific to a type of photography or subject matter or techiques. Look for things that most interest you.
And/or, you might take some classes or seminars... or get involved with a photography club... or sign up for a photo tour or safari. All these can be a great way to learn and expand your capabilities, too. Might be as good an investment, as gear would be.
I'd also suggest you set up a plan, with future gear upgrades or additions to work towards when budget allows. For example, maybe you don't shoot macro now, but would like to try your hand at it sometime in the future. It might be any number of things.... Justg start researching and developing a list of things you think you want to try or items that you want to improve upon. It might be a simple upgrade. For example, if yours is the 18-55mm kit lens that was originally supplied with the XTi, there are newer and improved versions in the EF-S 18-55 IS and EF-S 18-55 IS STM. The latter's "Stepper Motor" focus is faster, quieter and more accurate. An even more premium and, of course, more expensive upgrade would be the EF-S 17-55/2.8 IS USM lens.
Don't know how much you use it, but there are also a number of possible upgrades from the 75-300mm lens you've got. The more affordable is the EF-S 55-250mm, now also with STM focus drive. There are also several 70-300L IS USM is the most premium build and image quality. Any of the EF 70-200mm lenses would be very nice upgrades, too... these are real "workhorse" lenses you'll find in most pro shooters' camera bags. The IS versions of these are stabilized, which can be important with telephotos. The f4 versions are more compact and lighter weight, which might be nice for travel.
You may not have the funds for any of these now, but someday you might. And if you have a plan of gear you'd like to add or upgrade in the future, you can prioritize based upon your usage and needs, and start working toward the eventual upgrade. No plan is set in stone, either. You can always change it, if your needs or the available options change.
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & PRINTROOM
You are quite right and finally seeing the light. 
