http://www.xbitlabs.com/articles/video/display/radeon-hd5870-cpu-scaling.html
Pretty good article. Though I wish they included a C2D Quad cause you cant really compare C2D vs i7 and say its simply (dual vs quad).
But this line at the end does make it seem like i7 is the way to go:
Using this setup, I could quickly switch from the system with a dual-core CPU to the system with a quad-core CPU and feel the difference between them in the same parts of five games. Excepting S.T.A.L.K.E.R.: Clear Sky, I easily felt the difference such games as Tom Clancy's H.A.W.X., Resident Evil 5, Call of Duty 5: World at War and Crysis. The gameplay was much smoother on the quad-core CPU, without occasional jerks as on the Core 2 Duo. Game levels were loaded faster, too. So, the numbers don’t always provide the full picture, yet I hope that this test session will help you decide what CPU you need for one or two Radeon HD 5870 cards.