Prior to the TurboGL drivers the G400 kind of sucked for OpenGL, but they've always had top notch D3D performance.
Thanks to TurboGL a G400 (non-max) can keep up with a TNT2 Ultra/V3-3000, the G400Max slaps both around.
The G400 is a very good gaming performer. It's no GeForce2/Radeon, but it's better than a V3 or TNT2.