okay, so I compiled the 2.5.70-bk19 kernel, pulled my IDE HD out of my I850e (P4T533-C) box and dropped it in my I875 (P4C800) box with a Geforce FX 5600.
I booted in the new kernel, applied the patches to the NVIDIA_kernel module from http://www.minion.de/
and modified os-registry.c to turn on SBA + FW, compiled the module and started X.
I had a look at /proc/driver/nvidia/agp/card|host-bridge|status and it looks as though the card was running with AGP8X + FW, but no SBA.
I started up Wolf:ET and everything ran flawlessly.
The only bitch I have about the P4C800 is that the on-board 3com 940 gig-ethernet controller isn't support under the 2.5.70 kernel.