for 500$ you can get 2 6950 1gb cards or 2 560 ti 1gb gpu's in there. that will give you the best performance for 500$. and it should max out bf3. and your psu can run both cards
NewEgg recently (and may still have) Gigabyte GTX 560Ti (OC edition 900MHz) for $204 after rebate. A superb buy in my opinion. And yes, 750w PSU would be enough for two of these in SLI.