You're PC may be nearing the end of the performance spectrum.
It is probably ok for true classic games like PacMan or Space Invaders, but don't expect Mortal Kombat 3 to be running flawlessly.
I currently have a P3 800, 512mb of RAM w/ ATI Radeon x800 (256mb AGP) card, and it has serious difficulties with some of the more 'modern' games, including Mortal Kombat 1 & 2, even Street Fighter 2. Unfortunately.

I'll be looking for a cheap/free newer older PC once i finish my cabinet.