The performance gain is most visual when you go from 1 to 2 cores and from 2 to 3 cores. There are no point in going more than 3 cores if you are a gamer. Just check out reviews on Toms Hardware, they always get to the same conclusion.
Oh, and the reason i5 and i7 performs the same in this test is basically because you didn't feed it enough memory to let the cpu feast on its hyper threading and its 3 memory lanes. 6Gb is the recommended spec for an i7. But then you would have had to upgrade to 64bit, like ANY decent pc should run on anyways.
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