Yes, I managed to get 4 commercial LCD screens running off my system successfully!
I used a recent motherboard with 2 x PCI-Express slots, and 2 x Nvidia Geforce 250 cards, both with dual-DVI outputs. I also needed both the PCI-E power plugs coming from the power supply into the two cards. Installation was relatively easy and pain-free, and I now have 4 monitors appearing in Windows Display Control Panel, with the ability to choose different resolutions/configurations, and using the Nvidia control panel can even rotate screens 90/180/270 degrees as required. It works brilliantly!
The only remaining issue is that if any monitor cables are removed while Windows XP is running, it somehow senses that and the configuration stuffs up - Windows may disable that monitor, and it doesn't come back up automatically when the cable is reattached. Also, the order of plugging the monitors in (while in Windows) affects the numbering of the monitors. I would prefer that those monitors remained enabled even if the cable was detached momentarily (while testing our setup). Does anyone know of a way to disable that auto-sensing functionality? Or a way of ensuring that a particular DVI port is *always* monitor 1 or 2 or 3 or 4 etc.?
UPDATE: Have since found that Windows 7 is MUCH better than XP at handling multiple monitors, especially when the plugs are taken in and out at unexpected times. Win 7 usually brings them back in the same configuration while XP does not.