I don't know what type of motherboard you have, but most likely you've either got a few PCI or PCIe slots available. The cheapest thing to do is just go pick up 2 identical video cards and pop them in. If you think you might ever want to attach more monitors than what you have available slots for, then you can get a dual monitor video card which will only take up one available slot. As long as you buy all identical cards, you shouldn't have too many problems.
