I use two screens at work (as a software developer) and it's pretty much essential to my workflow these days.
I'd love a second monitor at home, but my GFX card doesn't have the support.
I'm not completely convinced of this. The monitors use their own power and the processing should be handled by your graphics card. There might be a subtle increase in chatter on the PCI bus, but negligible. Ultimately it's all about screen real estate - the configuration (big rectangle or two smaller rectangles) is neither here nor there.