I have 3 displays on my 2.66GHz Mac Pro: one 30" Apple Cinema and two 20" Dell 2001FP, one on either side of the 30".
The 30" runs off an nVidia 8800GT in the 16x PCIe slot and the aux displays run off an nVidia 7300GT in slot 4 (configured to 8x).
As several of you know, I'm a radiologist and I will often need to view cases (CT, MR, ultrasound, digital radiography) from home. I reboot in to Windows and power up the two Dell displays. The PACS software we use, Philips/Stentor iSite, makes use of 4 displays - and by happenstance it treats the 30" Cinema as two independent 1200 x 1600 portrait displays... essentially the 30" divided right down the middle.
This mirrors quite well the dedicated diagnostic workstations we have at the hospital, which uses two 21.3" LCD greyscale portrait FDA-approved 3MP (1536 x 2048) displays as the two center diagnostic-grade monitors (and 19" HPs for the two side non-diagnostic displays). The 1200 x 1600 images provided by each half of the Apple 30" cinema, with a properly calibrated color profile, come remarkably close in quality to the medical-grade displays.
So, at home, the left-hand side Dell displays the work lists and other text data, the 30" Cinema in the center acts as two independent displays for the images, and the Dell on the right-hand side acts as a display for the dictation system, 3D reference images, and any other windows I might happen to need up at the time.
When I'm running my machine in OS X, which is most of the time, the two side displays are off and disconnected (so I don't end up with windows trying to open on the powered off screens).