430ex or 580ex

What makes the 580ex better than the 430ex? Is that worth saving up

extra for? (I doubt it because the sooner I get a flash, the better,

but I would like to know your opinion.)

Another Question is, "Should I get a flash at all?" Do you think I should rely on the 20D's ISO and built in flash? Or do you think I should get a speedlite for bounce capability? What other fun things can you do with a speedlite?

If you shoot indoor events or need to have daylight fill capablity beyond the 3-5 feet range, then yes, you need an external flash. I don't know the price differencial, but 430EX looks like a nice upgrade from the 420EX, and may suit your needs.

The 580EX can act as a wireless controller, the 430EX can't. The 580EX has more power, stroboscopic operation and manual zoom control.

 

If you ever want to use a multiple flash system, you'll need a 580EX as a controller. The 430EX (or a 580EX) can operate as the slave.

 

See http://www.bobatkins.com/photography/eosfaq/ex_speedlites.html

In addition to what Bob said, the 580EX also takes an external battery pack like the Canon CP-E3. That's a pretty big advantage if you do high volume flash shooting where you need the flash to maintain good battery power and fast recycle times.

The best guide to the 430EX features that I've found is here:

 

http://www.eos-magazine.com/EOS%20system%20folder/EOS%20system/Speedlites/speedlite430ex.html

 

It DOES have the capability for manual zoom and manual power settings. Personally, I doubt the 580 is worth the extra unless you anticipate a very high volume of flash shots or need its extra features. If you later expand to wireless shooting, the 430 will be a good slave in the system. I would recommend you get an off camera shoe cord, and diffusers (Stofen or Lumiquest - also consider gels or "gold" white balance matching), perhaps a Better Beamer if you use flash with telephoto lenses, and a flash bracket. Those accessories will help improve your flash shots much more than the extra features of the 580 IMHO.

Excuse my ignorance but what exactly are gels?

and what's just about everything else you said....

An off camera shoe cord allows the flash to be connected to the camera while mounted on a flash bracket or while being hand held. A suitable flash bracket allows the flash to be positioned further from the axis of the lens (thus eliminating redeye which can still be a problem with longer focal lengths otherwise), and also making any shadows cast by the flash less obtrusive in the picture - particularly when you are shooting in portrait orientation. Diffusers soften the light, reducing the harsh shadows that direct flash can cause, and are particularly useful when you don't have a convenient white ceiling for bounce flash. Gels are colour filters that can be used to match more closely the white balance of other lighting in the scene so that you get a more natural overall result - mixed light sources such as fluorescent or tungsten lights and flash can make a real mess of your images. A Better Beamer concentrates the light from your flash so you get more reach when using lenses with an angle of view of a 200mm or longer lens on your 20D.

Do you know of a link where I can get more info on gels?