Ati Radeon x600 pro, PCI-E

Just a suggestion. Start by determining what you are willing to spend. Personally I have always been an Nvidia fan so I cannot advise you too well on the Radeon series. I also have the tendency to run "last years" card and do so for about an additional 3 years or so.

I would do some research on Tom's Hardware: http://www.tomshardware.com/us/#redir and look at review on sites such as Newegg. If you were going the Nvidia route and did not have a price ceiling I would suggest purchasing a card by BFG, and getting at least a Series 9. They have good support, and warranty even if the card is overclocked. They also slightly overclock their "off the shelf" cards.