First thing I'd suggest is try dropping the X4 955 down to an X3 440 and seeing if you can afford an HD 5850. It'll be a lot more powerful, and the CPU doesn't really determine the gaming performance.
Next, I would get a cheaper case. Check out the Antec 300, Antec 300 Illusion, Coolermaster 690, Antec 900 (or 902) and HAF 922. The last two might be more expensive, so don't use them if they are.
Then, I would get a quality PSU. Thermaltake isn't the highest quality. Check out some 650W units from Antec, Corsair, XFX, SeaSonic and Silverstone. If the budget is really tight, an OCZ 700W unit would likely suffice, but isn't a strong recommendation.