I have a 2x1.8G5, with a 6800GT main card (recently replacing a 9600XT) running 23" and 20" Apple Alu panels, and a 9200 pci secondary card running a 17" 1280x1024 Sharp lcd (needed a panel with a secondary VGA input to switch a PC through). Sometimes had the TV out on the 9200 running through a TV elsewhere in the room which I guess makes four.
The 9200 doesn't support QE by default, so I initially found having it active slows down the Expose/Dashboard transitions on all displays. If you force QE on the 9200, then Expose etc run fine, but large-area screen updates on the 9200 display are relatively slow; if I place a Safari window across the join between my 23" and the 17", spacing down a large document - eg, an LJ friends page - clearly lags behind on the half of the Safari window being output on the 9200 17" display. Playing avi/divxs, or an EyeTV output window on the second panel causes the video run at about half the normal frame rate. Reducing playback window sizes helps, but it's obvious that QE isn't coping well with the PCI interface. Effect seems true under Panther or Tiger.
Keep meaning to turn QE back off again, to confirm the lack of acceleration improves video/redraw performance, but well, I like my expose transitions.
Upshot, you can do triple/quad monitors, but don't expect too much from the displays on the PCI card.