If you take the GTX560 I would recommend you the 448 core version, and apply some overclocking, will give you a few more fps as long as your cpu does not bottleneck the system.

If you take the 2500K/2600K go for a good motherboard [Asus, EVGA, etc] and you can overclock them easily to 4GHZ and more wink

(Oh, and make sure that you motherboard fits in your case, and buy a quality-modular PSU wink )

MarianFrische.com