Re: Poor Xinerama Performance
When using TwinView (and performance is good), do you see this warning (or something similar) in your X log?
(WW) NVIDIA(2): The GPU driving screen 2 is incompatible with the rest of the
(WW) NVIDIA(2): GPUs composing the desktop. OpenGL rendering will be
(WW) NVIDIA(2): disabled on screen 2.
Adding a PCI card to the mix would likely reduce performance if you also had an AGP card because the PCI card is a bottleneck.