For 15 some years I used to make a point to build my own DAW's from whatever were the premium components at the time - but at this point if all you really need is a box that primarily is working with a single stereo track at a time (or maybe a few stem tracks at most) while doing processing primarily in the analog realm - using separate computers to pitch and catch - then even a few year old cpu's and systems will do just fine if not fantastically.
Honestly - the needs of computing muscle for a studio mainly doing mastering is actually pretty minimal compared to what those doing gaming or video editing (or simply steak swinging about how great their computer is) would choose.
i.e. - I'm just running E8500 Core2 Duo CPU's in boxes running Win XP - one of them in a self built box I made 4 years ago - and another in a second hand Dell Optiplex 760 that I got recently for all of $200 and then maxed out - and it all works great. Sticking with the older boxes and older OS also allows me to continue to use the Soundscape Mixtreme192 and Lynx One PCI cards that I've used for digital i/o for years - and since all of this works great and since "upgrading" to other cards or boxes would just cost a bunch of cash but not really gain me any actual additional capabilities - it makes more sense to just keep what I already have. The app I use (SAWStudio) also works just as well on WinXP as it does on later Win OS's - so there's no need to change this either.
fwiw - my DAW's are used only for work for my studio though - I don't even have them connected to the net (as I have uploading/downloading, net surfing, and Quickbooks all handled by a separate dedicated office machine).
Regarding the OS wars and people's either fan-boy type of devotion to Mac or their often defensive "but I can do that too" Windows advocacy - I have to strongly agree with John Moran - choose the applications you wish to use FIRST - and then use the OS that works best (or at all) for these. Meaning if you choose Sequoia/Samplitude, or need to run Plextools or other CD error checking software - you're going to be dual booting into Windows on a Mac box anyway.
Finally - I strongly agree with this following article regarding choosing electronic equipment which can be easily maintained and keeping it for as long as possible as not only a sensible choice but also the most ethical decision in these things - Unfixable Computers Are Leading Humanity Down a Perilous Path | iFixit
Still - for personal use computers I can totally understand the motivations for choosing the shiniest latest and greatest stuff so you can feel like you're the slickest when you show it off to friends. But for mastering - at this point you don't really need much power relative to what can be cheaply gotten if you do the "heavy lifting" outside of the box anyway.
Anyhoo - I'm typing this on a 4 year old laptop a friend gave me for free that I brought back from the dead and maxed out and have now running Ubuntu 12.04 - to truly "think different". I've been finding that the recent actually human friendly Linux distros are definitely fun OS to play around with - if not quite ready for general DAW use if ease of setting up hardware or the widest choice for DAW software is your concern though.