well if you have a few hundred to spend then id get one of these http://www.ebuyer.com/UK/product/124861/rb/26235257145

The Nvidia 8800 series cards are the newest breed of cards and the first in the world to support the new DX10 games(which are coming out soon and work on vista only) but they will play DX9 games aswell like half life 2, Battlefield etc etc, it will max all games out aswell

Whats your currant spec though? as you wont be able to take advantage of a powerful card like the one above unless you have a decent processor etc