Need help buying a PC.

[Untouch]

FYI the i5 sandy bridge will be faster than most i7s, especially setup for an OC. A couple MB less in the L3 cache but that's no big deal, the i7 sandy bridge is a touch faster though, at 3.4GHz.

That's wrong, most i7's will be quicker.
By increasing the speed (GHz) won't necessary make it faster.
You'll find that the bus can only take so much when you transmit data between the processor and memory that's why there isn't processors with super high (GHz).
Only computers not made for public with special buses would be able to cope.

That's why the i7 has 4 cores and 8 threads. It can transmit much more at once so therefore will be faster.
A processor with 4 cores and 4 threads with something like 3.2GHz will be slower than a processor with 4 cores 8 threads and like 1.8 GHz.