Guide number is a measure of how powerful the flash is. It's nominally the distance (in feet) that the flash can illuminate a narrow field of view to an acceptable exposure at certain conditions. The 580EX II has a guide number of 191, while the 430EX II has a guide number of 141. Both of these flashes have excellent features when used with a Canon EOS camera. Either one would be an excellent addition to your kit. They fully support ETTL2, so metering is automatic. These flashes' settings can be set from the EOS camera's menus rather than using the buttons and displays on the back of the flashes. They automatically zoom to match the focal length of the lens as you zoom in and out. They have tilting heads so the light can be bounced off of the ceiling or a wall for diffuse lighting rather than a harsh direct flash.
Maximum sync speed for standard flashing with the EOS Digital Rebel line is 1/200th of a second. The EOS xxD line goes up to 1/250th. However, either the 580EX II or the 430EX II will support high speed sync mode, which will let you get correct flash exposure at up to the maximum shutter speed of the camera. You'll have to turn on high-speed sync mode from the flashes' controls, and you'll lose some effective flash power, but with high-speed sync mode turned on, you can use as fast a shutter speed as you like with either of these flashes.http://www.rpphoto.com/howto/view.asp?articleID=1026
I have the 580EX II and use rechargeable AA batteries. Other than being quite heavy, it works extremely well and is very easy to use.