The main difference is the clock speed and the fact that one ending in K is an unlocked processor intended for overclocking. The 3570 is faster AND allows for overclocking, so technically it is the better chip. However, I don't know of any games where both wouldn't perform so well that they wouldn't put the limit back on the graphics card.
So, I guess short version: 3570K if you don't mind spending the money and wishto overclock, but both perform excellent for gaming.
You might consider the i5-3350p. It's a got a slower turbo boost than the 3470, but it is often quite a bit cheaper because it lacks the built in video of the other i5's. But since this is for gaming, we can assume you'd have a "real" graphics card anyway.
i5-3350p is $160 on Amazon right now, $40 cheaper than the 3470. Sometimes it's about the same price as 3450/70, though, so it depends on the deals that week. It's only worth it if it's cheap (like now!)
Honestly, all the i5's are all great for gaming. It doesn't really matter that much which one you get.