where to start

I think a mac has its own advantages.
Some of them:
A MAC comes with or you can obtain Itunes Music Server for free.
Connectivity: A Mac comes with USB and Firewire 800. You don't have to purchase a sound card or usb to Spif convertor.

While I have a PC setup, I would recommend going with MAC if you have the option of starting from scratch. MAC OS is just more stable. Why deal with the PC headaches (drivers, patches, compatability issues) if you don't have to. Only downside is that Mac with top end audio software will cost you more in the near term than PC. But you will more than likely have the more efficient platform going forward.

That being said, I am enjoying using my PC laptop with JRiver and JPlay.

Get a MacBook Pro but load your music on a firewire-connected external hard drive. Buy Pure Music ($129) as your music player; it sits on top of iTunes and works with it so you can play high-rez FLAC files through Pure Music which you ordinarily can't do on a Mac. Buy a USB-DAC (so many options--I chose a North Star USB DAC32) and a good USB cable (I chose a Wireworld Silver Starlight USB for $250) to run from your MacBook Pro to the DAC. Hook your DAC up to your preamp and do all your playlist creating, music selecting, etc. on iTunes on your laptop. You can also use the Apple remote app on your iPhone or iPod Touch or iPad. I have been enjoying great sound and easy user interface with this setup.
Alternatively, you can buy a Squeezebox Touch, connect it to your preamp (its internal DAC is really good) and stream all your music to it, using your laptop (running the Squeezebox interface, Logitech Media Server) or iPhone/iPad etc. to select your music, playlists etc. The Touch can also be used with an external DAC via coaxial or Toslink cables, but try its own internal DAC first, you may find you don't need an external DAC. I find the Logitech software interface really easy and flexible to use, more so than iTunes, but YMMV.

I hate Mac's. Used a mac mini for a little while and could not get rid of it fast enough. Cute looking box though....

@Edorr: You need to know how to deal with macs :P