Hi Sergino,
I run MIR with sibelius and it works just fine with my setup. However I run MIR and sibelius on separate PC's and communicate using MOL.
Looking at the specs for your computer I recommend you buy yourself an intel I7 PC, either a Intel Quad-Core i7-920 or something more powerful like a Dual Intel Quad core XEON 5520, depending on your budget. I don't think MIR will work under a Mac OS. MIR is such a processor hungry beast I wouldn't even want to try running Sibelius and MIR on the same machine, let alone try running multiple instances of MIR. Just loading up one hall profile takes up to about 4 GB of memory.
I've had no problems using the Sibelius soundsets I downloaded from the VSL website with MIR.