GPU is king in gaming, 2500K vs 2600k, 1600 vs 2133 is on playing with the marginals at best.
Except for the fact that this thread is about memory, I would almost completely concur. Though at 2560x1600 I have had to tweak with my north quite an amount to get the fringe and lace on my frames I have wanted. That said, I'm eyeballing a 3rd card early this upcoming year. Ebay special perhaps...
Gpu's do help the most. The op is not making a poor decision at all upgrading to 8gb from 4gb kit though. I saw a greater amount of a performance boost from the amount of ram than I have ever seen on speed upgrades.