Whatever brand you use, be sure that they're e-ttl compatible. In addition to Canon's own EX series, look into the Sigma DG-ST and DG-ST Super. The Super has radio slave functions similar to that of the Canon EX series.

Metz also makes an e-ttl-compatible flash. There may be others, but I have not investigated them. I settled upon the Sigmas purely for economy.

I'm getting satisfactory results with my Vivitar 285's (on a Wein Safe-sync)set to automatic. The drawback to using this very old technology (as you probably already know) is that the Vivitars often exceed Canon's recommended trigger voltage. (The safe-sync eliminates the possibility of turning your camera into a smoking flambe). In addition, the old technology is not as accurate as newer e-ttl, nor is it directly compatible.In order to use the old technology, you must set the camera to manual mode and set the flash corresponding camera aperture value.

I'm finding that when using the Vivitar with the 10D, I have to be VERY mindful of distances because the CMOS does not have the same exposure latitude as film. If I stay within the Vivitar's range, exposure is okay.

Be aware too, that only e-ttl and ettl-II compatible flashes will work on your 20D on anything other than the 20D's manual mode.