U basically need to upgrade everything for gaming, depending on te games. New games won't run very well on this comp. I would save some money and get a new PC.
For upgrading this comp, I would get more RAM, at least 256 or 512MB. Also upgrade your videocard.
TheMajorMMX - Intel P200 MMX @ 225Mhz - V-Tech (PcPartner) Baby-AT mb w/ Intel Triton TX chipset - Nvidia Riva TNT2 M64 w/ 16MB - 256MB M-Tec SDRAM - Still running.....