Hello, my current specs are:
Phenom II X4 980 BE @ 3.7 Ghz (I can't seem to OC it no matter how hard I try)
CoolerMaster Hyper 212+ Heatsink
ASRock M3A770DE motherboard
XFX Radeon HD 6870 Reference GPU
Corsair TX750w PSU
NZXT Gamma Classic case
I was thinking of using ~$350 to upgrade my GPU to the MSI 7950, but then I saw that upgrading to an Intel i5-2500k along with the ASRock Z77 Extreme4 mobo would cost roughly the same price. I'm wondering, which would give me the most performance gain in newer games like Guild Wars 2, Skyrim, BF3, Sleeping Dogs, and upcoming games like ACIII, Crysis 3, and Dishonored?
That's quite a good machine you have there already. But, if you're desperate enough then the graphics card is what I would upgrade. You'll see a much bigger improvement (benchmark wise anyway because you'll be hard pressed to see it in gameplay) than to upgrade the cpu.
Yeah the PC you have at the moment is very very good. I would leave it the Phenom 980 is still quick at stock but if I were to upgrade anything I would upgrade the motherboard to something better so you can get that Phenom up to 4ghz at that speed it really flies and worries (although doesn't beat) a stock i5 sandy bridge 2500k.
Alternatively, you could just drop in another 6870 if your mobo has a second slot (pretty sure 750W would be sufficient, but don't take my word for it). But unless your gaming at something higher than 1080P I would expect you don't particularly need an upgrade.