I'm waiting for the dust to settle with all my plug-ins, and C4 to mature, and still figuring out what to do with my 2 UAD-1 cards (integral to my workflow), before I make the plunge to a MacPro.
Currently, I'm stone-age... G4 Quicksilver733, MOTU 2408MkII, 2 UAD-1 cards, Mackie HR824 speakers, and Cubase VST 5.2pb as the main sequencer, and Nuendo 1.6 (I think) for mixing (love the automation!).
It seems prehistoric, but will happily track 24 tracks at a time from the MOTU, and playback 48 or more comfortably. The UAD cards are a life saver, doing reverb and high quality compressors and EQ without hitting the CPU. But primarily, it is stable, few crashes, and I can fly around on it...! (I am toying with upgrading it to a dual 1.8 G4 just to keep VST5 working).
The studio is a bit fancier (that I work at)... SSL, ProTools HD, Apogee converters, B&W monitors, two REAL LA2A's, two REAL 1176's (no UAD stuff here!) and Manley Slam and Massive Passives and hardware L2 for mastering. Plus a boatload of other toys... BIG FUN!
I've been on Mac OS for ten years or so (since a Starmax clone, later upgraded to a G3/300 processor) and before that, Atari ST4 (to run the earliest forms of Cubase).
But I have NEVER had a PC, and, after trying to troubleshoot some friend's systems, I will NEVER get one. If I need Windows, I'll use Parallels on the MacPro...
But no store closer than Birmingham (5hrs) or Atlanta (6hrs) to take a peek at the new stuff
You are a lucky man...