Ok. So the obvious upgrade (to me) is the second matching HD 5850 to run dual GPUs in XFire.
If you can download CPUZ, open it, then tab over to Memory.. the specs should be listed there. Adding 4GB to match @ $40 - $50 is better than 8GB @ $80 - $100.
You CAN go with a new CPU + MB, but your I5-750 is still pretty respectable. You could benefit from a mild overclock, but if you aren't interested in that stick with what you have at the stock speeds. The second HD 5850 will be your biggest upgrade..