As mentioned, main difference with the 430EXII is the stronger foot design and slightly revised control layout. Power is the same. I would take the newer one for a tenner, but not too bothered. They both do the same things.
580EX is a bit different. It has double the power (about a stop more) and can be used as a Master controller in a multi-flash set up. It also has a pull-up high-light panel which is handy. In addition, the 580EXII is modified like the 430EXII, plus it has an external sensor for use with non-TTL metering cameras.
All excellent guns though.
Edit: oops, crossed post with those above <must not make cuppa while posting>