Which CPU should i buy?

Recently i bought a new motherboard nvidia 630i. Now i am planning to buy a new graphic card "9600gt" & a quad core cpu. But, i am totally confused buying a best performance cpu. I googled many websites and i found a benchmark results between q9650 & phenom2 x4. The results says that x4 is little slower than q9650, that is only 2 or 3 points slower then q9650. Since, my motherboard supports intel 775 socket so i can buy Q9650 which costs $329 in india but at the same time x4 costs only $267. So, i thought that instead of spending $329 on q9650 i can buy x4 at $267 + $63 for x4 supported motherboard. Please help me which cpu should i buy???? & my another question is if i install nvidia graphics on a amd chipset motherboard will its performance decrease???