Our software engineers should chime in here. Amongst other systems, they all use new MacMini servers for their work, too. I don't know by heart how much RAM their machines offer.
That said, 12 GB is a realistic minimum requirement for any serious work, especially when you plan to use a DAW on the same machine. But that's more related to sample-based instruments in general, not so much to MIR itself. You can get pretty far with clever use of VSL's RAM Optimize feature, but that might not be the most convenient solution. The use of SSD drives seems to be the most up-to-date enhancement for these machines, because you can reduce the preload buffers to a mere minimum then.
HTH,
/Dietz - Vienna Symphonic Library