We saw a cool setup, six displays running on Vista, on a rather standard system. HIS demonstrated its PCIe 1X cards and we learned that if you plug two PCIe 1X cards and a single PCIe16X one you can end up with six display support.
The HIS PCIe
1X card is based on X1550 chip and it comes with 256 MHz memory. The card is
clocked 550 MHz core and 800 MHz memory and one link dual DVI ready.
If you need more than two displays this might be the cheap way to go and you are only limited by a quantity of PCIe slots.