I don't think there are many games currently available on the market that can really make use of the i7's full capabilities. Heck, I don't think even Windows 7 utilizes it to its full potential. No idea how long it will be until it is commonplace, safe to assume a couple years I'd say.
Gaming quality in general is going to be by far more heavily influenced by your graphics card moreso than your processor. I'd go with the cheaper 2500k.
Yep. 2500K. People generally go 2600K in the very best gaming builds because you can usually get a couple of extra bins of maximum overclocking out of it. You won't need that, though. The 2500K should OC plenty well with a decent cooler (check out the Scythe Mugen 3).
As far as gaming and depending on the title there is none or next to no difference between performance. Games are lightly threaded and respond much better to frequency than to multiple threaded architecture.
I have a 2600k and will gladly disable the hyperthreading to overclock my chip to reduce heat and gain extra frequency to try and speed games up. Too bad with my current chip it responds with the same bsod at the same frequency heat or not with hyper threading vs non hyperthreading.
I only replied with an overclockers perspective because you are looking at "k" class chips.
I5 2500K is the unofficial gold standard for a Gaming Rig CPU and even it when OCed is overkill for just gaming. There is really little difference between i7 2500K and i5 2500K other than money wasted between the two and HT on the i7 which games don't use anyway. From what I have seen in some game benchmarks the i5 2500K out does the i7 2500K all others are a wash funny enough but really the i5 2500K is a Cadillac stop thinking you might get more from an i7 cause all you will get more of is a lighter wallet.