Here is a way to look at it. I think this is how your viewing it anyways. Sacrificing a better GPU for a better CPU or vise versa. when compare the two cards alone. there isnt much difference. the difference between the two GPU's are SLIM. (482 cuda vs 512 cuda) only about 20 cuda difference. I'd go with the I7 2600K if you can afford it. You'd be more Future Proof. oh and make sure you have a SLI ready Board and Power Supply. Then you won't really have to get anymore Graphics for a few years. By SLI
i5 2500k is close to the i7 2600k in gaming so no need to cost extra money.