The i5 750 is the best bang for buck gaming CPU available. All else being equal, if its a i7 860 with a decent GPU or an i5 750 and a better GPU, the 750 will win (for gaming) every time. Get the i5 750 and spend that extra on a GPU upgrade unless you're already adequately spec'd out. If so, just save the money. So, what GPU are you planning and what are the rest of the parts spec'd out?
Heres the two head to head. i7 860 in blue on top, orange is i5 750. The gaming benchmarks are near the bottom. Exactly the same except Fallout 3, the i7 has a 4 fps lead, but at 86 vs 90, you're not going to know there was a difference. http://www.anandtech.com/bench/default.aspx?p=108&p2=10...
well...an i5 750 is very good but when playing with a very strong graphics configuration and high resolution the higher stock clockrate of the 860 make give it an edge if you dont overclock. If you do then it doesn't really matter. And hyperthreading doesn't really make a difference in games.
Take a quick read of my design guide in my sig. It walks through my thought process on designing a rig. You'll see for gaming, I start with the monitor and go for balance from there to the GPU then the CPU. I haven't updated it in a while, but the principals still apply.