Haha! I was asked this exact same question yesterday, so I did quite a bit of research, and here's the gist of what I found out:
i3 = dual core, it allows hyper threading ( = better multitasking), however i3 processors do not support turbo-boost, so they do not automatically over-clock. This can cause a slight performance difference. So under some conditions, i3 performance might go down to lower level Core 2 quad performance (note Core 2 quad is better than Core 2 duo), while i5 600 series and i5 700 series are equal to mid level and higher level core 2 quad processors.
i5 = support turbo boost, the dual core i5's have hyper threading, the quad cores don't. Performance is rather better than the i3.
i7 = quad core, rather expensive, support turbo boost, appear as 8 cores due to hyper threading, generally better performance than i5.
Go for the i5 750. Its supposed to be nearly as good as the i7s.
Edited by NightWarlock, 16 May 2010 - 10:11 AM.