What they said. Get the i5, overclock it down the road if you need to, and use that money towards a better GPU if this is a gaming rig. If you're going to be editing HD video or have applications minimized in the background while you do intensive stuff, get the i7. But it's really only if you plan on doing truly intensive floating-point and algorithm calculations, other wise it's just an e-penis enhancement.
On the other hand, if you have the patience I would wait until October/November, when the next-gen systems come out and Intel/AMD roll out their new platforms. The bar should move quite a bit by then, and even if you aren't getting the cutting edge at that point an i7 will probably cost what an i5 costs now. This is a move I'm recommending in my guide-in-progress.