If your absolute, positively only concern is gaming, then the i3-2100 is more than enough. Hell the pentium G620 is enough. But if you want to branch out into video or photo or 3d rendering? The 2100, 2500, or 2600 are appropriate upgrades.
first priority is gaming.. but, i also want the best performance including the speed, graphic and ect for others work.. that why i choose i5 2500k.. concerning the graphic card, does XFX 6870 can run the ultra setting for highest games like crysis, battlefield 3 and more?
For a mid ranged budget you're super! Your 600W Corsair PSU is more than enough to cover the hardware installed. The XFX HD6870 is very good also. You may not be able to play with Ultra settings on all games but I think you will be more than happy with it!
CPUs do not affect gaming fps by much , if it doesn't bottleneck ur gpu , then ur Good .
the only component that matters in gaming is the GPU , with a 6870 , ull get 28~32 FPS in Bf3 with AA Disabled @1080 , and ULTRA setting , in crysis 2 , DX10 without hires pack , ull get easily 40 FPS , but with the Pack and DX11 u'll have 6~10 FPS drop @ 1080 ...
if u can , get a 6970 MSI Lightning or a HAWK 6870 , since they're both OCed , they will be morepowerfull than a Stocks , and ull be able to OC them even more ...
If you intend to overclock your processor your going to want a better cooler, that one is hardly any better than the standard stock heatsink from Intel/AMD, and actually worse than AMD's better stock heatsink.
If you have no intention of overclocking, I would drop down to a cheaper I5 and invest the extra money into a better GPU.
If you still want the CPU power but want to downgrade, you could try getting the I5 2400 - it's enough of a CPU cut to perhaps upgrade the GPU. You could also stick with the stock cooler and upgrade the GPU further.