Well, on the AMD side, your pretty much only option is to go with the HD4850 option. Its in the annals of $200, and has GREAT performance. On that, you really have to pay attention to what core is used on Nvidia's cards, as they have more or less become part of their nomenclature lately. On that, an 8800 with outperform a 9600. On that, and again, it all depends on the core used, but for the most part Nvidia's 9 series cards, are re-badged 8 series cards.
NVIDIA GeForce 9600 GT Performance Preview