Well, from a practicle point of view, if the primary function of your PC is gaming (no photo/image editing or 3D CAD), the i7-2600K is a definate overkill. The only time you will see difference betwee the i5-2500k and i7-2600k is:
- Synthethic graphical benchmarks
- the intergraded intel video HD3000 benches
- under-load power consuption (about 7W difference)
Best you check out some Intel i5-2500K vs i7-2600K reviews and make your decision; HWC has one:
Intel Sandy Bridge Core i5-2500K & Core i7-2600K Processors Review
EDIT: of course, if you are going to get a 2600K, then might as well go all the way and get a 2700K.