Intel Ivy chips are coming out in a couple months. They are heard to be priced the same as today's chips so consider waiting.

The difficulty of software optimization is a further reason why adding more CPU cores doesn’t help much. Game developers have made progress in using multi-core systems, but the rate of advance has been slow. Games like Rage and Battlefield 3 — two high-profile titles that use multiple cores — both utilized new engines designed from the ground-up with multi-core scaling as a primary goal.
-Lax
The way i7 cores work is 1,3,5,7 are physical (real) cores and 2,4,6,8 are logical (fake) cores. My understanding is that for optimal performance, each fake core should be paired up with its corresponding physical core. And you should never assign only a fake core.

With 5 WoW instances on my i7, here is what I set mine to:
Slot 1: 1,2
Slot 2: 3,4
Slot 3: 5,6
Slot 4: 7,8
Slot 5: 5,7 (this one is assigned 2 physical cores)

This has served me well and is what I recommend to other i7 users. Enjoy