Try to figure out what you need the card for, then see some benchmarks. Here's an example:http://www23.tomshardware.com/graphics.html?modelx=33&model1=581&model2=586&chart=210
I read in several places that nVidia cards do better than ATI cards in some games, and worse in others. If you have a favourite game it's probably best to pick a card of the appropriate type.
If you're just watching movies and playing Solitaire you probably don't want any of those cards, go for something passively cooled like X1600XT.