It depends by what you are you doing in the computer, the i7 have hyperthreading and the i5 does not. The i7 800 series, and the i5 doesn't support triple channel, and the i7 900 does.
The i5 is basically the i7 without hyperthreading and I think it is a good value for the money.
If the computer is for gaming the i5 is really enough
i7 for photo and video editing, or higher-end software applications (which hyperthreading will benefit). OR if you plan to Xfire or SLI two double-GPU cards (like 2x HD 5970 or 2x GTX 295). These massive video cards benefit from all 16x PCI lanes, most GPUs will run fine on 8x PCI lanes.
See the sticky at the top of this page about how to ask for build advice. Let us know what you plan to use this computer for, how long you intend to keep it before your next upgrade, and how much you are will to spend.