Hi Dave,
You should be fine running Sibelius on XP 32bit, thats what I use. I purchased my MIR computer via CyberPowerSystems and then upgraded the memory from 4Gb to 12Gb and installed an RME Hammerfall DSP soundcard myself. I also purchased my DAW/ Sibelius machine via CyberPowerSystems a year earlier, the specs are: AMD Athelon 64 Dual Core Processor 5200+, 2Gb of ram.
CyberPowerSystems are not a specialist company for buildng DAWs, but I have found their pricess to be very reasonable.
I have installed an RME Hammerfall DSP soundcard in my DAW/Sibelius machine also and I route the audio via ADAT from my MIR machine back to my DAW computer and record the results within Cubase. I've also installed an RME analogue audio out expansion card for my Hammerfall card in my DAW machine so I can hear the results via my Monitors.
You will certainly need a soundcard in your MIR machine, I'd recommend one of the RME ASIO sound cards. MIR does not stream audio via the network like Vienna Ensemble 3.
I've had some good results with this setup
I hope this helps
Chris