i was planning to try out a triple head as you setup first then perhaps get a second TH2G for for the 4th screen, i don't believe my LCDs support 57hz,all depends no how well the gtx260 can keep up with my usage...
the Xinerama bug is here: http://bugs.freedesktop.org/show_bug.cgi?id=18668
for the testing i did for that had 3 screens hooked up to attempt to isolate the bug as initially i wasn't sure if it was a session boundary or gpu boundary issue. thus far it appears to be directly triggered by an animated cursor passing over an X session boundary. Its much more likely to occur as graphics load increases as its more likely the cursor rendering doesn't leave the animated cursor code path before it crosses a boundary.
a common use case for me is winXP in vmware full screen on 1 monitor. 2 windows that are run under wine and are 3d accelerated (pcb/schematic software if i'm working or 2 x eve-online clients if i'm playing) then "support windows" spread about, firefox, pidgin, gkrelln, pdfs, amarok, etc. Not uncommon for me to have mplayer up with a movie on while working as well. Under this load i'm not nearly cpu bound but 2D performance suffers terribly. QT apps like amarok (note: i use gnome not kde) become a complete slide show and firefox scrolls in lock step on some sites (slashdot) and i have to be very careful with added windows, a couple firefox windows and a few pdfs open and the system starts to have intermittent 'hangs' meaning the screen locks up for a few seconds but the processes are not locked up, its purely graphical.
it also appears that with this setup performance degrades over time, after a while i could close almost everything and mplayer can't manage to play a movie without stuttering or occasionally playing in a pulse like manor, i.e. the framerate isn't constant but "wobbles" tho the audio doesn't resulting in desync, etc. closing everything but mplayer or even restarting mplayer does not fix this. I have to reboot X.
my usage is odd in that i tend to run a couple apps using 3d/opengl that aren't terribly stressful by modern standards and a ton that are 2d only and not using opengl.
180.22 has improved the situation to what i described above, with 177 i couldn't get away with half as many windows open..