"Our tests demonstrate fairly little difference between a $225 LGA 1155 Core i5-2500K and a $1000 LGA 2011 Core i7-3960X, even when three-way graphics card configurations are involved. It turns out that memory bandwidth and PCIe throughput don't hold back the performance of existing Sandy Bridge-based machines. "
http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106-4.html

So I'd look at bumping the RAM first since that's easy, cheap, safe to do.

The next round is that GFX but read http://www.tomshardware.com/reviews/gaming-graphics-card-review,3107.html and pick what you allot to this upgrade. You'll have to check if the card fits and your power supply is big enough for the machine and new card.
Bob