Fostot,
Welcome to the forums.
It looks like you have a pretty good handle on the equipment issues inherit with multiboxing on a single pc.

I suggest splitting the WoW's up on as many drives as possible. Something I did not do when I started 5-boxing on one PC (and still have yet to move them) and I think I am seeing very noticeable lag due to this... my comp will kind of freeze up and I hear the computer hitting the HDD pretty hard.

Ram Choice: You went BIG... I like that. I have only 4 gigs (taking up all four slots sadly) and plan to upgrade to 8 sometime in the future.

I suggest, if you can afford it, the third monitor if only for the relief to your eyes when trying to read anything on the alt toon (without having to do the PiP shuffle).

I think your best bet is to build the best rig you can afford and go for it. That is the only way you are going to be SURE... but I think what you have outlined will be excellent for four or even five boxing.

Again, welcome to the forums.