1) A PCI video card that supports dual monitors -or- a 2nd PCI video card
2) Two monitors
3) And the will to use them.
Multiple monitors are very easy to arrange on a B&W. The stock video card doesn't support dual monitors, so you need a better video card, or a 2nd video card. The easiest way is to use a Radeon 7000 or Radeon 9200 PCI card, both support duals and either one is a nice upgrade from the stock card. But adding just about any old PCI video card to a spare PCI slot will work too.
I hope you don't try to work dual monitors from the iMac DV in your signature. The B&W can do it, that particular iMac can't.