HD7850 or HD7870
*make sure it's a 2GB model and NOT a 1GB model
The following card is just above your price point but I thought I'd add it in as it seems a really good deal:http://us.ncix.com/products/?sku=69829&vpn=FX787ACDFC&manufacture=XFX&promoid=1084
The Sapphire linked above is an excellent card too. It's also overclocked so the performance difference between it an the HD7870 I linked might not be that huge.
HD7850 2GB (or HD7870 2GB on sale)
*ignore old-gen GTX500 or HD6000 series.