It is a bit tough to compare system unless you are using as many of the same components as possible in each one. Else it becomes rather irrelevant. I think a better comparison would be something along the lines of a Core2 Quad and a Core2 Duo..actually, here is a link to a gaming performance test based on CPU's.
http://www.guru3d.com/article/cpu-scaling-in-games-with-quad-core-processors/9
The link itself takes you to page 9 of 11 (the page where they review Crysis). You will see that the E8400 (dual core) performs as well as all of the other quad cores (aside from the Core2 Quad Extreme). This is one of the main reasons I went with the E8400 for my build, it performs excellent in gaming and is also easily overclocked--I'm running mine at 4.05ghz 1.3v
The review is over a year old, but is very helpful because the test system is not subjective--meaning they use the same exact components whenever possible, simply switching the CPU. Obviously for the AMD side, they had to swap more than just the CPU, but you get the idea.
It is a very well written article about the scaling of CPU's in terms of game performance, and you will see that there is very little benefit to a quad core in most gaming applications.
Bare in mind, this article was put out before the i5 was even a rumor, so do not expect to see results comparing the Core2 line to the Core i line.