If ya got about 400 bucks I'd recommend the Radeon X800. Just ordered a system myself with it installed, looks to be an awesome card! Make sure you have a processor speed of 3.2 or higher or you wont get full capabilities out of it. GL on your search.
Accessmirco.com is a good place to start
Newegg.com is very good site too
tomshardware.com is a good place to learn about different products, and has reviews on them, very good source of info.