You are surprised by this advice. Why would I jerk a guy planning an esoteric RAID around by recommending excessive RAM? He is obviously aiming for the elite.
My personal experience is that the Mac Pro can use about 12Gb with everything that i use switched on and rendering. Usually two Archicads, Artlantis, Cinema, Photoshoppe stuff, etc.
And any ordinary day it hovers around eight.
I put sixteen Gigamegz in for plenty of headroom. It is cheap. 8x2Gb i think.
Dwight Atkinson