I have an old Pentium III that drives 8 displays through one dual-head AGP GeForce2 MX and three dual-head PCI GF2 MX. Eight monitors in 2x4 (horizontal) are good for marketing displays and 4x2 (vertical) for Karaoke bars.
However for computing purposes, whenever I connect more than three displays to one machine, I find the cursor gets lost a lot, and I have to call home often. Three 1024 displays is more than enough real estate for me.
I used to drive a Heisenberg, but whenever I'd glance at the speedometer, I'd get lost.