obviously its not easy to give a sufficient answer if the question is asked indifferently ...
one computer will do the job with MIR and VI on the same system - for those who want to switch between both setup types.
can both applications run simultaneously?
it looks like even this is possible but because for lack of need we didn't test this with large arrangements (in both apps at the same time). remeber: VE3 receives MIDI and delivers audio over the network functionality, MIR relies on a local audio device.
the i7 ... the range of models currently goes from 920 (2.66 GHz) - 965XE (3.2 GHz), a 950 (3.06 GHz) and a 975 (3,33 GHz) actually with a new stepping is announced ...
to be on the safe side our test system has the 965XE and we couldn't bring it to its knees with average arrangements and number of plugins. of course a 8 core XEON 55xx system would be even more powerful ...
what i would never do is to put 16 GB into any nehalem based computer since the triple channel memory controller always needs triples of memory modules for best performance (6, 12, 18, 24, 36, 48, ect GB RAM) - for very best performance (= to run with 1333 MHz) actually only 1 memory module per channel
hth, christian