Depends if you just want it for your Windows desktop or for gaming. If it was just for the windows desktop then you could stick a second graphics card in your computer and enable the extra outputs for other screens. For gaming though, you'd need to use Eyefinity from a DX11 ATI card.
I'd play games on my main screen but the other two would just be for general use. Could I just stick my ATI X1800 in then?