i5s are the best bang-for-your-buck processors it you're going to use it mostly for gaming. Currently, I would say go with Haswell if you can, as they perform 6% better than IB, and 18% better than SB.
Haswell processors are currently Intel's highest performing Core series generation. Although they might not OC as high (in sheer numbers, i.e. clockspeed), they perform better per clock than the previous generations.
4.2GHz on a Haswell processor is about equal to 5GHz on an SB processor (of the same lineup, i.e. if they both are i7s).
The difference between the i5 and the i7 is the amount of cache, and threads. i7s have Intel Hyperthreading, which lets the processor use two threads per core, allowing higher performance in multi-threaded process.