First of all, Metro2033 is probably the worst example here, because it only cares about the GPU, just sayingNot to big of a difference from i3 to i5 fps
http://www.anandtech.com/bench/Product/677?vs=701
Diablo 3
http://www.anandtech.com/bench/CPU/506
Metro 2033
http://www.anandtech.com/bench/CPU/338
...
I just had a massive Planetside 2 battle earlier today involving an insane amount of people (dunno, perhaps hundreds with infantry,armor and air force combined), and well, let's just say that there are times when even 6 core wouldn't be enough