I have been out of the computer game for awhile but Nvidia is still the brand to stick with. The 8800 series is the way to go as of right now I believe. You will want to make sure the card is DX10 compatible. Here are some good websites to do some research on.
www.tomshardware.com www.anandtech.com www.hardocp.com
Everything you need to know can be found on these websites.