Minesweeper or solitare is what you can run pretty much. The CPU is slow, and I bet you've got a socket 478 system, meaning that you can't upgrade to anything faster than a 3.4ghz prescott pentium 4. You have a chance of running some source stuff if you get a new graphics card. AGP cards (I'm assuming your system uses AGP graphics) are really cheap on ebay, a good AGP card (geforce 7800) which will complement your CPU pretty well can be found for £20 (you'll need a beefy(ish) PSU, I think 350watt with 18amps on the 12vrail was the minimum). If you can overclock the CPU, that will help a bunch. But you would need decent cooling, and decent powersupply and a decent motherboard to do this.
but to be honest it's not really worth it long term. You'll be able to play games from 2006 and back decently, but thats about it (It would run Oblivion though). I would just look at one of Maximum PC's $500 gaming budget builds and build your own.