Uses for multiple monitors:
Video and picture editing, website design and development, .NET development, remote and local linux server management (KVM as well as VNC remote desktop), documentation writing, research, dozens of virtual machines and browsing as well as gaming (center monitor, I don't have a video card to span across all that space, its something like 4320x2100 all told).

Mourngrym, Feb 8 2010

Config 8946
Win7, Linux, 4 monitors, 2 video cards
Still using my Core 2 Duo Q6600 Quad Core machine. 8gb DDR2 memory, RAID 0 2x1.5tb OS drives, 4x250 video scratch and 2x1tb external (eSATA) data archive.
Card 1:
PCIe, Nvidia 8800 GTS chipset
NVidia Linux drivers are not full featured. If you use rotation, you lose compositing features with multiple monitors. NVidia needs to fix their drivers, if they can make it work in windows, should be easier in Linux.
Card 2:
PCIe Nvidia GeForce 8400 GS
Same as listed above for linux.