Ivy Bridge is about 8% faster clock for clock than Sandy Bridge. So if both chips are at 3Ghz the 3770K would be roughly 8% faster.
Ivy Bridge generally will not overclock as high as Sandy Bridge at least not with cheap air cooling. Yes this is due to difficulties with heat dissipation.
4.5Ghz is really the sweet spot that most of the Sandy Bridge chips can reach with a good air cooler. Ivy might be 4.4Ghz. Unless you are running more than 2 very high end graphics cards going higher than that is unnecessary anyway.
If I was buying new I would get the Ivy chip.
If you are building a gaming computer you should be looking at the i5 2500K and the i5 3750K anyway since the only advantage the i7 chips have is Hyperthreading and no games use it. Get an i7 if you use software than can make use of Hyperthreading like rendering and editing software.
I would go Ivy as well. It isnt a whole heck of a lot faster than Sandy, but its still faster. Its not that much more expensive either.
Now it wont overclock quite as well but youre talking differences of a couple MHz, maybe 3 at the most. You can still get very strong overclocks out of an Ivy Bridge processor.
I also agree that if this is to be a gaming computer, go with the i5 and put that extra $100 towards a better GPU. The gaming performance, not to mention most other applications, the i7 really isnt all that much faster than the i5 and certainly not enough to justify the $100 premium. Unless youre doing very heavily threaded work a lot and need the extra abilities, save that $100.