Originally Posted by jModels
radeon hd 4850 sapphire 512MB or asus 8800gt top 512MB
AMD Phenom II X4 955 Black Edition or c2q q6700
Strictly performance, the HD 4850 is significantly better than the 8800GT, and the Phenom II X4 955 B.E. is significantly better than the Core 2 Quad Q6700 in most applications (particularly gaming), the Q6700 comes close in some encoding applications, but over the P2 X4 955 B.E. beats it.
Here's an example benchmark showing the Phenom II X4 955 B.E. vs a Core 2 Quad Q6700.
You can probably find more benchmarks around, every one I've seen shows the 955 > Q6700, which is to be expected.