I have always been a fan of Nvidia, so there, I am biased.  :o~

The price/performance sweet spot of Nvidia (right now) is a GeForce 8800 GT or 8800 GTS. They give out nearly the performance of even a 9800 GTX ($210) but are smaller, use less power and don't get as hot.

Newegg has a 8800GTS for $160;
http://www.newegg.com/Product/Product.aspx?Item=N82E16814143119

I would suggest that particular brand (BFG) as they have Lifetime replacement for their cards!

And, obviously, if you want...you can double that budget and get one of the newer GTX 260's, which are much more powerful.

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