I was asked to get a computer for Sibelius so I asked about and got very different advice.

I build one myself with a silent fan and power supply with the usual 64bit processor and 1Gb of registered ram.

Only thing I did get different was a EMU 1212M music card which I think is overkill and has way too much to mess around with (takes up 2 PCI slots).

With the software and a dummy keyboard, speakers etc... and a Sibelius computer keyabord (which cost £80 and has a apple on the back) it cost us about £1800. The computer itself cost about £700

I steer clear of it myslef. Except when I play with it after school pretending to be Tongy with the DJ software that came with the card.