Just ignore the "Requires DirectX 8" message box. It works fine with NT4 and SP6. My system is a Dual PII-450 with GeForce2MX and this game rocks. No performance problems at all. Smooth as silk at 640x480 and looks great! Obviously Serious Sam 1 works fine too. Get these games they are both cheap and awesome. Haven't had so much fun since Team Fortress Classic. If you have a cable modem these games are awesome for fun CO-OP play, probably the best games ever released that work with NT4. All you need is a hot OpenGL graphics card and you are in serious business!
http://www.croteam.com