A great flash recommendation for your EOS Rebel XTi would be the Speedlite 320EX.
The versatile new Speedlite 320EX features an LED light on the front of the flash body that can be used to illuminate nearby subjects in dim light. The light is ideal for video recording but can also be used as a modeling light or as an AF assist beam during Live View shooting.
A flash release function further facilitates wireless flash shooting by enabling the shutter to be released remotely from the Speedlite 320EX with a two-second delay, which gives the photographer time to re-aim the flash if necessary. The Speedlite 320EX combines rich bounce functionality with high performance, ensuring smooth, professional-looking results.
Flash coverage and guide number can be changed manually by extending or retracting the flash head. The Normal position provides wide coverage equivalent to a 24mm lens (full-frame) with a guide number of 79 (ft.) / 24 (m) at ISO 100. The Tele position boosts the guide number to 105 (ft.) / 32 (m) at ISO 100 while providing coverage for lenses 50mm and longer (full-frame).
You can learn more, as well as find a dealer, here, on the product site for that flash.
Something I'd recommend with any of the flashes you're considering is to use them with an off-camera shoe cord and a flash bracket. This moves the flash off to the side and higher, farther from the lens' axis, so that redeye is less of a concern and shadows are nicely thrown down and behind subjects.