Well what motherboard, graphics card and power supply do you have?
If you have a newer Nvidia (well I think it's 7900gt and later, not sure) card and have a motherboard that supports SLI then yes you can depending on your power supply.
Same goes for crossfire, if you have a ATi (I think it's HD 2900 but not sure again) or newer and you motherboard supports it then yes.
But tell us what your graphics card, motherboard and power supply is first.
EDIT: Plus for future reference, SLI is what Nvidia calls it and CrossXfire is what ATi calls it.