I'll jump on the Apple Airport Express stump, since "amey01" brought it up. I'm probably the biggest Apple evangelista on this forum, and I've probably been using the Airport Express longer than anyone on here through my stereo, so I feel qualified (n obligated) to tell you about it...
If you already are using iTunes, and IF you had enough foresight to rip all your CDs to iTunes using Apple Lossless format at it's highest setting, and IF you have a decent stand-alone DAC (or a receiver/preamp with good on-board DACs) with a TOSLINK port free, then RUN (do not walk) to your closest Apple store (or Best Buys) and get an Airport Express and a good TOSLINK cable,and make sure you get one that either has a "mini-TOSLINK" plug on one end or a "mini-TOSLINK" adapter--you need that to plug into the Airport. (If you can't find a cable that comes with an adapter, buy a regular TOSLINK cable and PM me--I have a half-dozen extra adapters in my parts drawer)
Once you get your Airport configured (which is REALLY easy--just follow the directions that come with it), just plug it into a power strip closest to your DAC. Connect the Toslink to the Airport using the end of your TOSLINK cable that has the "mini TOSLINK" plug, by plugging it into what looks like a mini headphone jack in the Airport. Plug the other (standard) end of the TOSLINK cable into your DAC. The open iTunes on your computer and choose your Airport as the "output" (lower right hand corner of your iTunes screen). Voila!!! You are now streaming your iTunes library, WIRELESSLY, into your DAC, and getting it pumped into your system in all it's digitally-converted glory.
If you DON'T have a DAC, your only option is to us one of those 1/8" stereo mini plug--to--2 RCA adapter cables and using that same port on your Airport, run it into an "aux" input of your preamp/receiver. The Airport will be doing the D/A conversion, and it's pretty dicey compared to a quality outboard DAC, but it's better than nothing, and still lets you stream wirelessly into your system from a computer...
I can't recommend the Airport Express enough. If you're already using iTunes to manage your digital music on your computer, it's the easiest solution to your problem. No need to convert your files to something a Squeexebox ot Transporter can understand, no need to copy all your files to another hard drive, it's pretty much a plug and play solution! And if you have an iPhone or an iPod Touch, you can use them as a remote for iTunes, using the free app "Remote" from Apple's app store. It is very simple, very functional, and the price is definitely right...
The Airport Express is under $100, and it also can be configured as a network repeater AND a wireless print server if you hook up a printer to it's USB port.
http://www.apple.com/airportexpress/
The Transporter and Squeezebox are excellent products--don't get me wrong. The DAC in the Transporter is VERY sweet-sounding, and it offers a LOT of very cool flexibility and sophistication, AND it has those cool animated VU meters in the display. But it's a two thousand bucks (or more if you buy a big dedicated HD array), and although it IS compatible with Apple Lossless files, to get the best performance, you'll have to copy your library to a dedicated HD that you buy extra and plug into the Transporter...
If you need remote control, that's gonna cost you extra with the SlimDesigns gear too. Their remote for the Squeezebox or Transporter is VERY slick though, and has a LOT more functionality than using an iPhone to control iTunes--but it's also a LOT more complicated to use, and it's one more remote that you have to have lying around...
http://www.slimdevices.com/pi_transporter.html
I hope this helps. If you have any more questions about integrating an Apple Airport Express into your system, just ask. I'll be glad to help if I can.
--Richard