http://www.the-digital-picture.com/Reviews/Canon-430EX-Speedlite-Flash-Review.aspx
The 430EX has a Guide Number of 141' (43 m) while the 580EX has a Guide Number of 190' (58 m) (both at 105mm). The 430EX is not under powered for many uses, but the 580EX's significant power advantage can become important when using a flash modifier such as an umbrella, softbox or diffuser - or bouncing light off a high ceiling. In addition to being more powerful, the 580EX allows 1/1 (full power) to 1/128 power in 1/3-stop increments while the 430EX allows power settings from 1/1 to 1/64 in more granular 1-stop increments.
The other primary difference is that the 580EX can be used as a master to trigger remote Canon wireless slave flashes while the 430EX can only act as a slave flash.