$750 is pretty much for a GPU+PSU upgrade. You could get a great corsair PSU like this:
Corsair TX650 @ $89.99:http://www.newegg.com/Product/Product.aspx?Item=N82E16817139020
As for the GPU, I saw a lot of problems being posted with regards to the 670 so please research more about this option just to be on the safe side. Admittedly, the 670 is the best theoretically & it is hard to recommend something else if the OP can afford it. But better to be updated about any issue that may arise out of the recommendation & the you make a well informed decision.