I agree with Hell_storm2004. You need to have a better PSU for your setup. The PSU might be fine now, but might be an issue later (when you have more expensive parts running). I like Antec/Corsair/PCP&C/Seasonic PSU's for my preferred choices. I'd look for at least 400-500w PSU's from one of the brands I listed previously. If you get around 500w, you'll have more upgrade room later. Don't forget to get a PSU with at least 1-2 PCI-e power connectors too. That way if you get a higher end GPU that requires 2 x PCI-e power connectors, your covered.
If you are staying with a 1280 x 1024 resolution, the 4850 GPU should be just fine. If you upgrade your monitor to 1650 x 1080 or larger, than you should upgrade the PSU and GPU to accomodate the bigger resolution.