couple things
1. you would not need 8GB of ram if you build the clients right, now if you are trying to run auctioneer on 10 clients for some reason then yes lol
2. 4 monitors setup would be easy. 3 clients on each with 1 for your "main" or since you have two parties do 4-1 1-4.
3. that machine is way overkill unless you want to run all 10 clients at 50fps or something. the newer quad cores would be fine. wow is not very CPU heavy, nor graphics heavy.
4. hard drives, I used to be the person that runs one client from its own folder until I tried it different. You dont need that. For any client that you want unique (which would be your two 'mains' make a seperate folder. Then have one folder for the rest and launch a seperate instance, it really is faster and since they will be seperate accounts your macros wont get messed up (or anything else) then just put those folders on a different drive (both mains on one, alts on a 2nd) and you definately dont need 150gig raptors either 1 install of WoW = 10gigs.
5. would I want a computer like this? sure, would I ever build one? heck no because it is way overkill and overdone than it needs to be
6. for displays I would do 4 22" widescreens, they are big but not too big. could do 24" if you wanted for your alts to give you a little more real estate.
7. @shigan, this board indeed has 3 PCIX slots. however only 2 of them are 2.0 which the video card listed requires. so you wont have 3 top of the line ones in there no matter what.