U R right, Cyril... Hope that Apple is no more delaying the new mac pro because of waiting till the Ivy-Bridge Support will be released! I expect that they will release the new one in March 2012 because the pressure from the public eye is too high, so there could be an upgrade to the Ivy Bridge later this year, but they have to release the Pro with the Sandy Bridge and two thunderbolt slots. The new one will have Ram slots with 1600 MHz, but they are not offered until now from the manufacturers..., so there could be an expensive price-range at the start... An apple employer said that it needs more than 1 year till prices of new Dimms will decrease, so in case of that it is better and more economic to use the 1333 MHz Dimms, the chipset is using 4channel support for Dimms, so the speed between 1600 MHz and 1333 MHz is not the most important thing relating to a higher price-range.
In case of using multiple instances of MIR and VE5 in Logic, I think that 64 GB RAM bundled with SSD's over Raid and 2 x 8 core processors is more than enough to get a satisfying workflow for a longer time. Actual logic supports 12 cores, but in practical use there are lots of bugs by using the 32bit-Bridge, so Apple's Logic-Team has still a lot to do! I hope that Logic Pro X in an expecting PRO-DESIGN will break those limits!