Well, the CPU is slow, but so is the memory speed. I would personally say it's time for a new computer. You could probably hang on to the 4600 as it's not too bad, then upgrade the mobo, then proc to a 2.8 or 3.0, and get 512-1GB of PC3200.
But for the cheapest price/performance upgrade, I'd think it would be investing in a 9800 pro. You'd have to compare them though. I don't think a 2.8GHz (assuming your motherboard supports it) would offer as much FPS as a 9800 pro. But I certainly could be wrong. Check review sites.