Ever since installing the 9.13(?) forceware and stereo drivers I can use only my integrated graphics chip (Nvidia 6100) when I install the 7950 GTZ (which I bought just to use with the Z800!) and I boot up, I can see everything until just after the Windows XP screen. Where there should be the logic screen it's totally black, nothing is visible on my monitor or headset beyond that point.

It's not just my 7950 either, I tried some old (and one newer) graphics cards I had laying around and none of them would boot up into anything but pitch blackness.

How are people using graphics cards with these drivers when I can't? What's the problem with my setup? (Windows XP SP2, Athlon 64X2 5200+, tried with Geforece 8600GS also, pitch black every time)

The problem is probably your card or the cpu. It's a 7950 right (works well with 3d) but it's also a GX2 (dual gpu's which are internally sli'd). There could be that sli only causes problem for those who want to use two outputs from the card but there might also be a possibility that sli-config itself ruins 3d. Since i only have a 7900gt (works great) and a 8800gtx (still not working nvidia way dual projectors) i cannot really tell how the 7950gx2 performs with 3d.
Dual cores and 91.31: Some has reported success using dual cores with the 91.31 drivers while i have other experiences (all but one core needs to be disabled).
The 8600GS also sucks regarding nvidiastereo.

Some things to try:
First: As always as some wise man says: Say your best pray!
Second: If you want to use the 91.31 drivercombo then disable all but one cpucore. Nhancer does the job and works smoother than making a dual boot option.
Third: Try tinkering with compability modes, especially disabling sli and see what happens. I have no idea if this even is possible but give it a try.

You could also take a look at the iz3d-drivers since they're not grinny about newer hardwares. Winxp/vista dx8, dx9 and almost any graphcard will do. They don't support shutterglasses but manage anaglyph, dual outputs, planar modes and some others good. Anaglyph/iz3d is free while the other modes are on trialtime.

These are only my tips. If anyone else has, please post.

deactivate "Onboard Video Device" in your BIOS

edit:\ before that, deinstall video drivers