Regarding playing files stored on a USB hard disk, many people report success with this method and I wouldn't discount this option. I cannot tell any difference myself between a file accessed internally or externally. To attach a USB drive without AC power requires 2 ports, then you need another port for the USB DAC and with a macbook you only have two ports available unless one goes with the mac pro, which is way more expensive. For this reason, if planning to use external USB storage, consider a mac mini. The specs of the cheapest one are fine for audio streaming and it has 4 USB ports. (You'd need an external display and a mouse for the initial set up, though.) Control could be via iPod Touch - by the time you add the cost of this though you're nearly back to the cost of the cheapest macbook. This if fine if your music collection is small enough to fit on the macbook's internal drive, but even then it's still prudent to have external storage (or NAS or something like Time Machine) for protecting your collection against disk failure.
One advantage with using a mac is that the hardware and software are made for each other and audio is bit perfect by design. With Windows, there is no message that says 'Great, you've got bit perfect audio!' so unless you have a DAC that indicates that it's receiving a bit-perfect signal, there's always a nagging doubt as to whether you've really bi-passed the k-mixer.
At the moment I use a 2003 Toshiba Satellite running Windows XP Home SP2 as a dedicated music player. It is not attached to the Internet (I use another machine to rip CDs and get the song titles etc.), so there is no need for Windows updates and no anti-virus programs or other unnecessary services running. I run it in safe mode with network support and video accelaration off. The native processor is a bit too fast so I slowed it down to keep things cooler so the fan does not come on at all during audio streaming. Using JRiver media jukebox with the asio4all driver, this is outputting bit perfect audio - AFAICT! - and everything is very stable. 1 GB of RAM. I use an ingenious wireless remote system: when the album finishes playing, I get up off the sofa, walk over to the machine and select another one. While I'm not a tree hugger, at the moment I'm quite happy to extend the life of an old machine in this way. Too many computers as landfill in this world already if you ask me.
The main drawback with Windows is the interface. iTunes is really pretty. On a PC I'd stay away from iTunes as it's a large installation that installs services without the user's permission. I'd try foorbar, MediaMonkey and JRiver and see which you like best. They're all free.
Reliability is important with streaming audio:
http://www.computeraudiophile.com/blown_tweetersI don't have experience with the T61 but if it's anything like the older Thinkpads (I'm typing this out on an X40) be careful before using it as a music server as the older Thinkpads are notorious for beeping loudly at random intervals, even after you've set them up not to.
