I do one reasonably decent computer. I have 2 monitors, but usually run all instances on one monitor and have the second one for watching video, reading websites, etc. It definitely gets strenuous on my neck turning back and forth when I play; heck, even looking up at the minimap can become obnoxious which is why I have my minimap overlaid atop my center of screen.

The advantage of single machine boxing is one less thing to maintain, I guess. And my current PC is pretty efficient - it draws maybe 75% of the power of my old machine, so 2 machines would likely (even if lower spec) draw more electricity. I don't really see any disadvantages to single machine boxing as long as you have the hardware to handle it.