you really dont have much to game with in that system. You definitely need a new GPU and to do that you're going to need a much better power supply. And the worst part is the best processor you can run is a phenomII x3 so a quad core is out of the question. Not that an x3 is bad but it will leave you behind when games using quad cores become more mainstream.
in the meantime. do a virus sweep, spyware scan, hijacker scan...LOL