Since you have the money there is no reason to not, if you don't have the money, and want to concentrate more on your GPU, then core 2 quad will be the way to go, if you wanna save 200 - 300$ going core 2 quad, thats fine, but you won't have the same upgrade path for the future.... you'll have to throw out your ddr2 ram and get an entirely new mobo/cpu
All in all for you i'd recommend going core i7, but also what do you want to do with it? Gaming? What resolution do you plan on playing at? Theres a lot of variables to your question