Right now I'd wait a few months for the new direct X9 cards to come out ( don't think any are out yet - although I haven't been keeping up to date.) If you really must buy a card now pretty much anything with 128Mb+ with do perfectly, some people report problems with non GeForce cards. At the end of the day both types are probably as bad as each other =D
I'm running an old Gainward GF4 Ti4200 golden sample. Never had any problems.