750 Watt should be enough for 2 GTX 550 Ti in SLI. However I am not sure if you´d be able to overclock the cpu too, perhaps should be not a problem, depending upon the speed you push the cpu, and PSU brand.
I think I have to agree with Jemm. I think a second GTX 550ti in SLI would be the most cost effective performance upgrade.
Good suggestion Jemm.
Agreed with above^ Maybe not the best performance boosting upgrade in game play but an SSD's. They have came down a bit and with some good deals out there it would make things feel and load a little quicker...