If you have any intention of overclocking the CPU, the answer is definitely the 2500K. If you leave everything stock, the i3-2100 offers more bang for the buck.
Keep in mind, too, that you're using the CPU whether you're gaming or not and that GPU prices fall faster than CPU prices which also tips the scales toward the 2500K/HD6870 now and a 560Ti six months from now for something like $189.