A computer is no longer fast when it no longer suits your needs. So if you're having doubts about it, then maybe you do need a faster computer. So... If you don't need that one anymore I know someone who does
J/K, you're computer is fast enough to run all the games that I play. However if you want to get into more intensive 3D games, then upgrade your vid card. That'll breathe new life into your system. I built a P3 800 with a Radeon 32DDR and I pretty much just play Starcraft on it. (well, Day of Defeat too)
I will undoubtedly build a new computer but mine has yet to become obsolete.
/me drools over the prospect of next system being 2ghz