Guys, Half Life runs smoothly on a P5 166MMX with 32 MB RAM and a 4 MB 3Dfx Voodoo. The same probably goes for CS. I'd recommend at least a 200MMX for Unreal Tournament, though.
The computers Gekko has are a major overkill for Duke 3D and Quake. Duke needs a 486-DX4 on 100 MHz to run smoothly, though a Pentium is recommended for higher resolutions. Quake runs perfectly in 640x480 on a P5 133. None of them support 3D acceleration.
If you're desperate for a multiplayer driving game, don't play GTA 2, and especially not GTA 1 (because GTA 1 runs horribly on anything faster than a P5 166, due to a poorly implemented frame limiter) - play Carmageddon 2. It runs well on anything stronger than a P5 133 with decent 3D acceleration. Carmageddon 1 is IMHO a bad choice, because it supports 3D acceleration only when patched, and even then it works only with 3Dfx Voodoo. On the other hand, without 3D acceleration it's so ugly I can barely stand to look at it.