If you have the budget, go for the i7-2600k; if your budget should need to go a little lower than that, get the i5-2500k, because it's still very overclock-capable and still very powerful.
This is IMHO, but: As a gamer, unless you're looking to throw money away, you shouldn't really need more than the i7-2600k. You could go as low as the i5-2500k (especially if you aren't utilizing processor-intensive media programs ... while multitasking out the wazoo), unless your intent is to compete in benchmark/stress tests or to really lend a hand with Folding@Home projects. Again, you won't really need either of the latter if you're a gamer, and the i5 and i7 chips are MORE than capable of artsy programs and multitasking.
There are other factors like the number of RAM sticks and the speed thereof, and what kind of VRAM you're toting on your GPU, but looking back at this thread seems to imply that the rig you've been recommended has such grounds covered -- I don't think there will be any concern for future-proofing there!
tl;dr: I would honestly say go for the i7 or the i5 (but make sure they're the Sandy Bridge generation of Intel CPUs!). If worst comes to worse, the motherboards they've recommended will mean you are good to game in terms of CPU, GPU(s), RAM and audio for a good few years to come, and even beyond because they should still be able to support the next several years of the upcoming iterations of processors and graphics.