With 5k you can buy 1 kick ass machine for about 2k for your main and 4 other computers for you alts :P

If you really set on having one machine you could use a tri-sli board with 3 x 9800GTX (or even a lesser GPU), however since these boards are made for SLI and generally for playing one high-end game at the time (such as crysis) not sure what kind of issues you may run into running them independently, if any. I think the tri-sli MB is the only way you would be able to hook-up 5 monitors.

If you go the one computer route let us know how it goes.