I personally find the mega-pc is not really going to cut it for you.
The main reason is memory, there is a limited amount of memory bandwidth just as there is a limited amount of GPU bandwidth.
I use two pc right now to run 5 copies of wow, I can run all 5 on one pc but I find it laggy. I almost never lag on two pc.
I just wonder if you couldn't buy 3PC using much less expensive hardware to get your 10 copies going. You loose the PIP but you get so much more in other ways.

3 pc have 3x the memory bandwidth of a single mega PC.
3 pc have 3x the GPU-PCI bandwidth ( as in data going to/from the video card via the PCI bus )