As far as I am using, it is best to use Intel make MB (recommended Core2Duo) and Processors. Regarding OS, there r several choices like, Fedora, SuSE Linux, Ubuntu or CentOS in which Fedora and Ubuntu is free. Though u r using for basic jobs, the server is not required here.

Emperror Linux , SYstem 76 are Linux Based Machines. But, bit costly.
My personal opinion, Dell is OK with avg cost.

You can get with NO OS installed on them. Then Install Mint or Ubuntu for easy to use, user friendly things.
After installing a few minutes of internet use will satisfy your Music needs. Office suite will come along with Distro

I have never owned a Mac. I use Mint 12, am very pleased with it, and would highly recommend it. It offers a choice of GNOME or MATE interfaces. MATE is more user-friendly but still is under development. One thing about MATE, you have to customize it to use double-clicking. (This is can be easily done by any novice but just thought you should be aware.)

Try linux mint 12 with codecs.

I too am a Mac convert and I have always been a Debian user, even when I was a Mac guy I always had a Debian box. If you like OSX's interface (minus the fancy eye candy doo-hickies) you can configure gnome to look and feel just like it in moments and it is a very intuitive. If you want eye candy install compiz, I don't use it but it is pretty. It comes with a built in cd ripper, although I use ripperx. I use gramofile to rip vinyl, etc. If I need to do any polishing, editing or cleaning to ripped audio I use audacity. Debian is maintenance free, as are most distros, and your only real learning curve will be package management, which is a new concept to anyone converting to linux. I have been using Debian full time, 100% for absolutely everything since 2006 and my only concern, the only thing I miss is all the money I wasted on Macs.Linux does it all, and more, and it does it better.