I never get rid of my old puters' but dual/tri/quad/multiple monitor cards? are they really optimal...isn't it more effective to just have separate cpus? also if one puter' breaks...your not screwed?
I would love to have 4 of these connected to my switch! (that's right! a 19" laptop with a detachable screen! by my friend samsung ) http://www.dynamism.com/m70/
You would need separate licenses for each CPU and will need to do a lot of file transfers to work with the multiple monitors that way. This is not to mention the cost of the hardware, power requirements, etc.
Duals are very common. Quads if bought used are affordable and reliable. After all, they are for professional workstations and deigned for longevity.
one mouse/keyboard controls multiple monitors = very important. kalvokar or whatever Switch not fast enough as well "Always on top" nvdia screen management very useful. As long as your computer have enough memory, definately dual card is much better then seperate computers. I run a laptop + a computer with 3 lcds. in case one breaks down. I have both running at all times though.