Well, to your first question, I guess all we can tell you is try it and see. Try to match up spells with similar cast times/effects so you can minimize the actual buttons.

And for the other question, it depends on what you want. I run everything off my laptop and an external monitor. I keep the active window on 35 fps cap and the inactive at 15 fps. If you feel you need more FPS then I would highly suggest different computers. There is the bonus that if you ever have a hard crash it's not an entire party wipe as you can move over to another computer.

If you're using Keyclone, I'm not positive, but I think he makes you pay for an additional license for other computers. So that may also be something to factor in. However when you get into multiple computers, there are other options other then Keyclone.