I can't believe that i'm saying this, but I have to agree with Dale. Bumping your RAM to 1gig with allow you handle anything within reason, and a good graphics card will really rev up your system.
I'm guessing that your mobo has an AGP slot instead of PCI-e? Make sure which you have, and then shop for a card. The 6600's have come down to a reasonable price, and the performance they give is pretty doggone good.
The bottom line is what your budget will allow.