In what way do you feel your pc is lacking?
It looks to me like you have a relatively well balanced pc.
If it is FPS for gaming, then the usual recommendation is to upgrade the video card.
In this case, the 5870 is already a good card, so a meaningful jump would be to a 7970 or GTX670 class card
If you are lacking in cpu power for your games , or whatever, the 2500K is a good jump.
To help clarify your options, run these two tests:
a) Run your games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 50%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
set to 50% and see how you do.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.
Unless one of those options do not clearly identify a problem, here are some other suggestions:
1) 4gb of ram is a bit substandard by today's standards. 8gb would be better.
2) A SSD is one of the best performance upgrades you can buy. It will cost <$1 per gb. The bigger the better for performance.
3) Buy a second monitor. The increased display area is wonderful for web browsing. when gaming, use one for gaming, and the other for e-mail, performance monitors, etc.