Its hard to say because nobody is benching games on 1440x900. They probably should be though.
What I do know is, my 5850 totally knocks out my 4770 crossfire in Crysis. Your cpu isnt slow, but of course you wont be getting the best results with it either. You will see a good increase in fps over your 4850 at least, I dunno if its enough to justify the cash however.