Most if not all the i5's are dual core. All the i7's are quad core. I don't think any of them are hexa-core.
What's your take on the AMD phenom II X4 955 black edition at 3.2 GHz?
Most of the i5's are dual core, but advertised at quads because of hyperthreading, making 1 physical core into 2 virtual cores. Any i5's with the name 6xx are dual, those with 7xx are true quad.
All i7's are quad, except for 970 and 980x, which are hex core
The 955 is a very good processor, it will perform as well in games and every day applications than your intel processors will, so for price if you are building a gaming/every day system, then it would be the better option, but if you are using CPU intensive programs, an i7 would probably be better because of better speeds clock for clock and better FP calculations. With the 955 though, make sure it is C3 stepping, not C2, as C3 runs cooler and overclocks a hell of alot better than C2
You can tell if it is C2 or C3 because of the name. Look at the last 4 digits of the product code:
IBOX = C2
MBOX = C3