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You'd think that a $5000+ project deserves more than a one-liner-post. What are you trying to do with it, what monitors do you consider, which software is supposed to run on it, ...?
Anyway, I'd wait until the fglrx officially supports Eyefinity and the Eyefinity edition is available, so you can attach 24 screens to 4 video cards. Not sure when that'll be.
Going with 12 2-output-GPUs will lead to trouble as there's no ATX-motherboard that can handle 12 PCI cards. 4 2-output-cards and 8 TripleHead2Go is still more expensive than Eyefinity and doesn't have driver support (so you'd have to manually configure Xinerama stuff and composition wouldn't work right)
If you can actually do it depends on your money and craft skills. Consider a good projector as a cheaper alternative - it has a lower resolution, but no black borders around the screens and it doesn't require exotic software setups.
Cards with 4 DVI outputs exist already in form of ATI FireMV 2400, but it is not exactly cheap. In combination with a 6 slot PCIe x16 Board like the upcoming MSI 890FX-GD70 you could drive 24 Monitors too.