Ok, this should be your camera (tell us if it's not): http://www.usa.canon.com/cusa/support/consumer/eos_slr_camera_systems/eos_digital_slr_cameras/eos_di...
This is the digital rebel (there were also film-based Rebel bodies as well.)
All EOS "Digital" cameras are compatible with Canon "EX" series speedlites. Note that there was an "EZ" series speedlite which is not compatible those haven't been made in a long time.
Eliminate the trailing 0 in the model number of a Canon speedlite and that is its "guide number" listed in meters. This is an indication of how far away a subject can be and still have adequate light for a proper exposure WITH THE BASELINE of ISO 100, and f/1.0.
With this knowledge, you can tell that the Speedlight 430EX II is stronger than the 320EX, and the 600EX-RT is the strongest flash in the lineup.
Your lens surely does not support shooting at f/1.0 (Canon made an f/1.0 years ago -- but not today) but the standard is used because it makes it very easy to do the math for any other f-stop by dividing the guide number by the f-stop. E.g. if shooting with a lens at, say, f/5.6 all you do is divide the guide number by 5.6.
Suppose you use a Canon Speedlite 430EX II. This flash has a guide number of 43 meters (that works out to about 141 feet). At f/5.6 you divide the guide number (you can use either the meters or the feet - doesn't matter) by 5.6.
141 ÷ 5.6 = 25 (approximately).
This means your flash would provide enough light to adequate illuminate the expousre for a subject 25 feet away when using ISO 100 and f/5.6.
It's nice to have a little extra power because it allows you to bounce the flash off a white ceiling or wall, or use a light modifier to soften the effect of the light. Bouncing and modifiers eat a bit of the light output from the flash.
Tim Campbell
5D III, 5D IV, 60Da
