Ok My tuppence...

I 5 box on one PC, on one monitor. It works and PiP is great but it would probably be better on 2... bear in mind aspect ratios if using keyclone and PiP. Widescreen is 1.6 so you would have to replicate that aspect ratio on your clone windows in the second monitor... if both are the same size, no probs the clones are just 1/4 the size.

Hard Drives
There si absolutely no point going th emultiu drive route with WoW... 1 HD will give better performance as the disk reads are cached at the disk and probably L2 which will blow the benefits of parrallel drive access. 1x access from disk and 4x access from cache is a lot faster than 5x access from disk.
NOTE: this does rely on using multiple parse points see junction or mlink [Other] PowerShell script to create symbolically linked (mklink) copies of WoW

The biggest bottle neck on my system (athlon x2 4600+ 2.8Gb RAM Radeon X1950Xt) is the CPU bore cores run flat out when running 5 instances, I am not approaching the memory limit and my gfx card seems fine. Therefore I would definitely advise getting the best CPU / Mobo you can.

Why 8Gb? This seems to be a bit unnecessary as 4Gb is more than sufficient to run 5x WoW, a video running in another monitor, half a dozen tabs open in forefox and an excel sheet open.