5850 is damn good. I've oced it too 900/1200 mhz and I am able to max out crysis 2 on 1680x1050 or crisis 3d at 720p. So that should give u a good idea. Glad to help.
Secondly for 550w u can either power a card like 69xx or 5xx. If u insist on crossfire then I would suggest picking up a midend card. For best comparison u should read the tomshardware best gpu for the money articles. They cover an indepth price to performance comparision. Lastly always buy a card meant for one step higher performance. This will ensure futureproof and allows u to max Out. Eg my card was meant for 1920x1080 but I run it at 1680x1050. Hence it helps me to play even the newest games at high details.
I only run 1280x1024 and am going to get a 5670 because it is double the power of my overclocked GPU, and il get another 5670 and crossfire them for even more preformance.