What are the benefits of having more than one graphics card installed on a Mac Pro? I know it allows you to have more than two monitors, but will it speed up graphics intensive work when its not being used to power a display? Thanks for your help. 
It does NOT help with games.
Yes it does.
Note that the description for the macBook Pro says 9600m GT but it also features a 9400m.
Ah, so you don't mean using CrossfireX or SLI.
Ah ok then, very sorry. It does speed up games if you SLI (nVidia) or CrossfireX (ATI) although I don't know if you can do that on a mac.