You're going to have to buy another video card. If your motherboard doesn't have an extra x16 slot, you can get a PCI or PCI-E x1 video card to run that that third monitor.
ok one more question , I have a crossfire motherboard , if I decide to run both cards @ crossfire will I be able to use that 3rd monitor , same with SLI . Thanks for the responses.
um,not sure if you meant the resolutions on the 2900 card using triplehead to go are limited
but the new ATI cards do the full(triplehead to go) resolutions