I'd recommend a HD5850 man. You'll be much happier.
I only get 20fps in STALKER with everything set to high (DX10 1680x1050).
I'm running a HD5850 overclocked to 775/1125. Could you imagine what a 5770 would get? 10fps?
Your running at 1680 though man, I've only got a 19 inch 1440 x 900 monitor. What other specs you got?. Really cant afford a 5850, wish that I could though