The 2500k is a great processor, both for gaming and for everything else. In fact, some say it is the best gaming processor there is.
The 3570k is the newer version of that which is a tad bit faster and with a tad bit more heat.
Because of the extra heat it overclocks less well than the 2500k, but because its a little more powerful at the base it doesn't really need to go as high for the same effect either.
Either one should work just fine, however, the 3570k would possibly need a BIOS update to be recognized by the P67 motherboard and the 2500k certainly doesn't need one.
It is personal preference which one you want, both are a pretty big step up either way.