After about a month of rebuilding the patched 8776 driver under my 440 Go, I stumbled across the tweak last night that might make it possible to use the 9631 driver on my card. This thread describes the exact problem, and the fix is a simple one-line option to the Device section of your xorg.conf

Option "UseDisplayDevice" "DFP"

I reinstalled nvidia-glx over top of my patched 8776 driver, and I got full acceleration from glxgears. I also tried it under a live CD of Xubuntu and it worked fine there too (although frame rates were depressed, no doubt from running the live CD and because of the added strain of the Xubuntu desktop).

According to the README for the 9631 driver. …

When assigning display devices to X screens, the NVIDIA X driver by default assigns display devices in the order they are found (looking first at CRTs, then at DFPs, and finally at TVs). This option can be used to override this assignment. For example, if both a CRT and a DFP are connected, you could specify:

Option "UseDisplayDevice" "DFP"

to make the X screen use the DFP, even though it would have used a CRT by default.

Note the subtle difference between this option and the “ConnectedMonitor” option: the “ConnectedMonitor” option overrides what display devices are actually detected, while the “UseDisplayDevice” option controls which of the detected display devices will be used on this X screen.

I should have time to try this on a clean installation tonight, which will be the true proof of it working. And hopefully my Feisty speed tweaks will be fully complete tonight too.