Which piece should I upgrade?

[Brewski]

I built my computer about a year and a half ago & I've loved it thus far. I'm thinking of upgrading one piece for the time being but not sure which would give me the most benefit.

Current GPU: Geforce GTX 650 Ti Boost
Current CPU: Intel i5-3570k

My main game is WoW so I'd want the performance to go up, but I also play other games on the side. Looking for some input.

Also just thought I'd add what I want to upgrade to:
GTX 970 or Intel i7-4790

[Arbiter]

Upgrade the GPU and overclock the CPU. Upgrading the CPU would require a new mobo which adds on extra price and won't improve performance by much. Overclocking it will give you the most bang for your buck. If you don't have an aftermarket cooler I'd suggest getting one.

[lolJon]

Definitely go with the 970. You can OC the i5 pretty easily depending on your motherboard (most of that needs a quick read of the manual)
You can also OC the 970 to get speeds close to a 980 (if you're looking for that)