Hello - rabid Mac guy here. First, lets debunk the whole thing about more programs. There are about 10,000 programs for the mac - maybe more. Safe to say that most of them are as specialized or useless as the 10,000+ for the PC.
MSFT, Adobe and all the heavies make their key apps for both, There are utilities to do everything for both. There are at least five browsers for the Mac... What the hardcore PC types don't know - and fail to tell you - is that Mac OS X is Unix based - so the cultural, philosophical influence is from Unix which is a truly enormous body of work and along with Linux where most of the talent is sitting. MSFT inspires no one
Now lets talk about Mac hardware. The key here is integration between hardware and software. besides which, till you meet him, you cannot possibly imagine anyone as demanding, as persnickety, as hard to satisfy as Steve Jobs. The man hates crap and he won't put his name on it
As far as what to buy. I would bite the bullet and buy new with the three year AppleCare warranty program or refurbished from Apple or Small Dog.
But if you insist... For audio only I would think about a late model G4 Powerbook - maybe a 1 or 1.25Ghz machine with Airport. Advantage is that its small, quiet and its always nice to have a portable. Stick with the Panther (10.3x) OS for another rev or two - in fact there is no real reason for you to go to Tiger to start.
If you have a nice DVI or VGA monitor you like you could start with a MacMini. Just spend the money to get at least 512 and preferably 1Gb RAM. You are in for a grand with the 80Gb HD, the bluetooth/airport card so you can sync your cellphone and do AirExpress and the extended warranty.
BTW don't worry about the fact that Mac is going to change to Intel. The Macintosh experience is about the software - the hardware is simply engineered and built to BMW standards because Steve wouldn't have it any other way - it will get faster in the future like it always has.