I have not personally tested the current 8/12 core mac pro's.
I'm currently running mac pro 3.1 as my main computer, and a 4.1 as slave (both 8 core.) I have a current 6 core mac pro running as a "tape machine" alongside a RADAR in the music recording room, and have played with / tested it in various ways.
Your question however, is "how long is a piece of string"... as I see it anyway. Even just saying an 11GB load... that is easy on one machine, BUT depending on your interface (internal on mac is MUCH better than external - although TB / PCIe boxes will change that!) and of course plugin / VI usage can change everything.
Every machine I have ever had I have maxed out. You just work within the limitations presented to you.
Personally, I'm looking into going the other direction - setting up a number of low cost machines that can be added to easily in the future if necessary. Small and light mac mini's which make for very easy portability.
New mac pro's are a while off yet. SB "E" is looking like november, so I don't see the new mac pro coming before then. It WILL be interesting to see what it is capable of - and if it indeed gets put in a 3RU case as rumoured. Still, I can fit 6 mac minis in a 3RU case, all in under 10KG... worth considering I think. The the idea of VEP5 running virtual effects racks is even more interesting when coupled with a scalable system.
All in one is great in theory, but I personally still think the idea of "scalable" systems will be with us for some time (at least for the life of a new system say.... peering into the crystal ball is still murky... but the current INTEL roadmap offers some interesting bits of information that show what workstations 3 years down the track will be like. I am personally excited, but then again, am always excited about tech. :)
Brendan.