Not really, unless you get a friend to pay you what you spent on the 2600. Try selling it first and see what offers you get. It could take awhile to find a buyer; it's tough to get your money back for any used components. They lose value faster than a new car.
What could you possibly be upgrading to? Really nothing you do would be an economically wise move. You already have a 5870, the only worthwhile upgrade from that would be a 6990, and to waste money on upgrading your cpu just to get a few extra mhz would not be wise if you ask me.
the 2600 is a perfectly fine CPU, it's still 300 dollars and has an excellent passmark score.
additionally if you are gaming, the majority of games are GPU intesive: http://www.tomshardware.com/reviews/sandy-bridge-core-i...
in the next page there is a CPU intensive game, your CPU will perform easily over the i5-2500k and the bottomline is games like F1 are few and far between. Worry about your GPU if you are gaming before your CPU imo.
depending on your motherboard you should be able to overclock the 2600 I think something like 3 bins (300 MHz) above the highest turbo. So I think it might do 4.1 Ghz? Someone correct me on the exact numbers, but you should be able to overclock it slightly. At this point probably not worth changing for the 2600K.
Also, stock speed is fine for the video card you have. Which btw is an awesome, and only slightly dated card. The weakness of the HD5000 series is worse crossfire perf than the 6000 series, but that's not an issue for you.