I would guess this cpu would equal the performance of a Althon x4 @ 2.2 or at best a little slower than a q6600?
Mabe someone can help me....
If you have a 4850/4870 series gpu, I dont think a gpu upgrade will help you.
Edit 2: take the below w/ a grain of salt... as apparently my reading skills are off today... did not realize that we were talking about an AMD 9550... my bad!
Something in his OP is wrong. A Q9550 is clocked @ 2.83Ghz from the factory. It is a faster part than a Q6600 at stock speeds and also has an extra 4mb of cache (12mb vs 8mb on the q6600). I also don't know how he got 5gb of memory (it can be done but very unlikely).
EDIT: should have looked at your sig (obviously you know what I said already). I assumed that his OP was just a typo... not that he had underclocked the part).
To the OP: I would stick with the 4870 (if that is what you have) for right now. Probably would give you the same advice for the 4850. Perhaps an upgrade would be worth it from a 4830 (given your price range).
I would say that an aftermarket cooler and some overclocking of that q9550 is in order if you haven't tried that yet (and your motherboard is OC friendly). You can push them well beyond their stock speed.