How can I go about doing this without directly going SLI? Can I just install a crappy video card to use as additional moniter support?
That will work (side note to be aware of, if you mix ATI+nVidia on Vista you can't use Aero, AFAIK). Matrox makes some video splitters as well, but if you're just wanting to add one extra monitor and the extra video card is an option, the price isn't really worth it.