I have yet to test with acpi=off, but here are my previous tests. All tests are run after reboot, "clean" system except for the applications running.
Geforce4 MX 440,
Athlon tbird 1.4GHz
Via KT400 chipset <--problem??
glxgears: normal window
tenmillion area=100 texture light zbuffer
glxgears 730.400 FPS
geometry rate: 488513 triangles/sec
fill rate: 48.851 Mpixels/sec
3656 frames in 5.0 seconds = 731.200 FPS
geometry rate: 486562 triangles/sec
fill rate: 48.656 Mpixels/sec
3671 frames in 5.0 seconds = 734.200 FPS
geometry rate: 488642 triangles/sec
fill rate: 48.864 Mpixels/sec
Note, theres virtually no difference between the drivers used for AGP (The difference you see fall down as statistical noise) and this would be the problem as I can tell it.
Bios is set as force 4x AGP mode (Linux has no agp8x support in 2.4 series)
So really, should there be no change between drivers, or is something quite messy here?