if your current videocard has both a VGA and a DVI outputs, you can do dual monitors today. if not, than you can go out and buy a ATi or Nvidia videocard thats PCI, PCI-Express, or AGP, as long as it has either 2 VGA, 2 DVI, or a VGA and a DVI output on it. You can use a DVI-to-VGA adapter and hook both of them to the videocard.
On my 6800GT, i have both DVI and a VGA outputs on it. i hook my primary monitor to the VGA connector, and my secondary to the DVi using the DVI-toVGA adapter that came with ym 6800. when i boot up my computer i had to install nvidia dualview, taht way, my 2nd monitor wouldn't just be a clone of ym primary one. when thats all said and done, i installed UltraMon so i could have my taskbar on both monitors and have panoramic wallpapers.
hope that helps!