You could buy a new mATX motherboard, a nice CPU, some new RAM, a new PSU and HDD......
1.7GHz Pentium 4 I think in that machine. You could likely stick some more RAM in it, and maybe even a new CPU. And a bigger IDE Hard drive.
But you might aswell just get a new PC because for the money you will get like a 10x faster system. Atleast.