My woes are neverending..
I put the system back together using the nForce3 board and an X800XT PE, and exactly what I feared would happen has happened: Unstable system.
This was exactly my experience with nForce2 boards, supposedly very fast but they are quirky and unstable as all fuck, so when the nForce 3 did the same I was not surprised but very discouraged.
System seems to work fine until I try to run 3DMark2001, it causes VPU recover errors and when I disable VPU recover it hard locks up.
It's almost surely a problem with the nForce3 GART driver and the way it uses the AGP Bus, possibly fast writes or something but I simply have no idea or will to troubleshoot this shit.
So unless anyone has any quick tips to try, I will tear this system down and rebuild it with the ASUS K8V Deluxe (K8T800) motherboard and hope it actually works this time around with the X800XT-PE.