that depends, are u getting a new one ??
after reading a few posts else were, some say a ATI, but I'm using a nvidia and its working fine, tho it is a 6800ultra....
depending on what else u are going to be using it for, depends on what card you get, tho if u are, get ether the ATI X600 upwards or the G-force 6600 upwards both are very good once above these two cards and u wont have to upgrade for a while at least, as there's not many games out there that push these cards to sweating point if u have them backed up with a fast CPU like a AMD64 3000+ or P4 3.0+
just remember, there's no point putting a monster card on a slow CPU (1.5ghz and lower) as u will just create a bottle neck of info because the CPU can not process the information fast enough to handle these new cards.
hope that helped, also as the 6600's are quite cheap now may as well go for a 6600GT unless u want to get a 6800ultra
arty: