Few games use more than 2 or 3 threads. The extra hyperthreads of a i7-920 will be largely unused.
Also, the 2500 is more efficient than the 920, by about 15% clock for clock, and the clock rate is higher.
If you get the "K", you can also get an easy OC to help things more.
If you already have a sandy bridge 2500k, there is no value in upgrading to a ivy bridge 3570K. But for a new build, I would go with some sort of an ivy bridge cpu. They are even more efficient per clock than ivy bridge.