If you end up going pc I would recommend at least a 667 mhz The price differential is not too steep.If you get a 550 I think you will end up spending more later because you will have to replace the motherboard,processor,and Ram. (because you are going from 100mhz to 133mhz system bus speed)
If you spend a little more and get the 667mhz
you will already have that 133mhz bus speed, and hopefully that bus speed will last you for a couple years. If it does, the only thing you will have to replace is the processor.
I have heard that macs are the way to go for music applications, but you're looking at a lot of extra money. If you have the budget make some calls, and try to talk to some professionals.