You can either buy another graphics card or buy a new 2 head card.
Windows will pick up a second card automatically and I believe you can extend the desktop across both monitors.
2 head cards are made by a few manufacturers, but Matrox seem to be the most popular. I run a Matrox Parhelia which gives me a resolution of 1975 x something I can't remember (!) on 2 monitors. I then extend my desktop across both (a setting on the driver) to give me huge working area.
Works really well - I cant imagine using a single monitor for editing again.