Go for a 512 MB 8800 GTS Nvidia based card.
Good card, I am running the Evga brand one.
If you get that, it should run the new UT III perfectly if you are into the UT games.
We have a server setup for the game
which I manage, and will soon be building a better server.
Stick with XP for games.
It is a proven fact that Vista slows down games right now.
I do run Vista on my new Dell laptop though.