Up memory to 512 as suggested.
Swop CPU for any recent Socket-A Athlon-XP or Sempron (up to the 2800 series at a guess). Socket-A is officially dead (though none of my 3 desktops know it) and consequently Socket-A CPUs can be found at bargain basement prices.
But you're above a certain minimum, games are far more dependent on the GPU/graphics card than the base PC, so beware. Best midrange bargain currently is the Nvidia 6600GT (much cheaper now that the 7800 series is out).
Or buy a new PC and install Linux on the old one.......