I've finally got my multi-head setup back. I went from a triple 21"@1600x1200 plus a single 1920x1200 to merely three 1920x1200 flat panel displays. It took a ton of doing and nearly eight weeks of effort, but I now have two nvidia 7800 GT boards driving two Apple 23 inch cinema displays plus a single Apple 23 inch Cinema Display HD. (Two of the new aluminum ones, one of the old plastic ones.)
Interestingly the two monitors are not the same even though they are highly similar. The problems arise using DirectX under Windows (x64 and normal 32 bit). When connected to the nvidia boards (using SLI and straight up) the newer ones work just fine while the older one refuses to run at anything other than 1024x768, 800x600 or 640x480. What makes this strange is that the old cinema display had no problem running games at whatever resolution I wanted when connected to my ATI video board.
When I sent the manufacturer a detailed description of the problem, their response was simply some pointers to forums about issues using the X2 cpus. Most of which seem to be driver issues relating to the motherboard. Here the video driver simply wasn't offering anything other than the three crappy resolutions listed above. Well, a ton of fiddling soon sorted it out. The lesson here being that you need to try everything in every combination before giving up. Even the stuff that doesn't seem sensical.
So, at the moment to maintain visual symmetry on my desktop, I have to use the right-most monitor to play games using DirectX or OpenGL. Thus far I haven't seen any improvement in game performance when using SLI versus not using it. Perhaps this is just the effect of not having any ultra-modern games. The most recent game I own that I've played is Battlefield: Vietnam and I can't even run that with everything turned up at 1600x1200. The game is unplayable at less than 60fps.
So now, I run Ubuntu with triple 1920x1200 screens and I'm loving the freed up desktop space from losing 225lbs of CRT. The only minor thing is that my overall resolution did drop a bit. I went from 6720x1200 to 5760. Enough room for another browser window or several xterms, but they were all too far to the right to be of great use anyway.
Now, I just need someone to patch twm to deal smartly with dead space *and* do smart window placement. Then I'll be truly happy.