1. It's the SOFTWARE FIRST
,the OS is (mostly) irrelevant.
2. figure out WHAT you want to do in music and select the software that fits your needs from there.
3. Buy the hardware that supports your software the best.
Frankly, in terms of just "getting shit done" Windows (7) is basically as good as Mac. Linux isn't so friendly, but if the software you need to get shit done is only on Linux, then, you're on Linux.
Now, there is a caveat with the software first thing, which is, your interface. If the audio in/out device you're using is Mac only, then you're using a Mac. Etc for the rest. So, for example for my home studio, I have a MOTU Ultralite MkIII hybrid running on windows 7 HP laptop. It's a bit quirky, but the sound quality is excellent and the preamps are smooth - for the price, it's hard to beat. There is better, but it costs more. Luckily, the MOTU is Mac/Win, and I happened to have this HP laptop not doing anything, so bingo: instant home music set up.
For software I run Ableton Live Suite - the fullblown monster. Why? Because what I do is more performance /composition based. If I was in a band and I was recording through some big multichannel interface, I would go with ProTools, because that's what I learnt in school, and it's pretty much the "MS Office" of the audio world (in more ways than one...) I also use Audacity, which is the swiss army knife of audio editing (i.e., small, crude, but effective)
For monitors at home I have a pair of EVENT PS8 monitors. They're a little bass heavy, but over all, very good sounding at a very reasonable price.
I don't use a mixing desk, I have an AKAI control surface and a Yamaha (XS6) synthesizer. Between them, I have plenty of ways of making things happen.
At work, things are very different - I have a ProTools C24 console and an SSL mixing desk with Bryston amps and Dynaudio 5.1 monitors and a Mac Tower running Protools, AVID, Audacity, Melodyne, Autotune, and a pile of other gear (compressors, processors, etc.) But that's almost half million bucks right there. So, "let's not go there" and let's focus on what you're trying to do.
So, get yourself an audio interface and some kick ass speakers, FIRST. Then figure out what software you need, and that will guide you to the hardware. When all is said and done, what computer you use is trivial, both in terms of effectiveness and expense. I bought my HP laptop (an old i5 running win 7) for $300 used. It works FINE. Ableton Live Suite literally costs THREE times as much. My laptop is one of the cheapest pieces of gear I own (my speakers were $650). So, don't sweat the hardware. Figure out the kind of music you want to make and proceed from there.
Here are some general suggestions
1. Rock Music: ProTools / Logic / Whatever - Focus on microphones and a good compressor.
2. Electronica: Ableton Live. Get a good control surface (I don't recommend Akai - mine sucks...) and a good keyboard
3. Dance Music: I would suggest a combo of FL Studio and Ableton Live
4. Composition: Finale and (whatever: Logic / Ableton / ProTools / Reaper / whatever) Your main point is to generate good composition - the software is just there to make it do something, so it will be more a question of what softsynths you use...)
5. Experimental: Cycling 74 Max/MSP or Processing. You'll need to get a Mac for that.
6. Jazz: See Rock.
That should get you started. DON'T TALK TO SALESMEN. They will try to sell you things. Things you probably don't need. Focus on what your interests and skills are, and then build your studio around that.