Macs with expansion slots have been able to run multiple displays since the days of the NuBus Macs.
Most modern video cards support two displays per card. Just load up enough video cards to drive the displays you want, then boot the machine. Really not much to it, actually.
Be aware that on standard PCI or PCI-X based Macs (everything prior to the second gen - PCI-express - G5s), only the AGP-bus monitors will get the benefit of Quartz Extreme-accelerated video. QE doesn't work over the PCI bus (not enough bandwidth).
I had three displays at one point running on my 2.5GHz G5 (two off the Radeon X800 in the AGP slot, one off a Radeon 9200 in a PCI slot). The display off the PCI bus was noticeably slower. This is no longer an issue with the current PCI-express bus.