Graphics card. Get something like gtx 670 if you can afford it.
This. That's almost enough money for a second 670. You could also crossfire 7950s (e.g. the twin frozr 7950 for $310 post-rebate).
Sandy Bridge-E is great and all, but it's not the best bang for your buck. It's not really supposed to be; it's for people with extreme tastes, not the practical-minded masses. 6 cores and all that PCIe/RAM bandwidth is not necessary or even particularly helpful for modern gaming.