I suggest you buy the receiver that best meets your needs without taking mp3/music streaming in to account. You can buy add-ons after that will most probably do a better job as separates than limiting yourself to a receiver that does all-in one.
The Squeezebox has been mentioned. Also take a look at the Roku Soundbridge. I have one connected with optical cable to my receiver. It connects wirelessly to the computers on our wireless network (wireless router needed) and accesses existing libraries. With iTunes, I've enabled it to share my music library, which is then accessed by the Roku Soundbridge and played through my receiver. To control what is being played there are two options. Either use the remote control supplied, or download and install software that will control the Roku from your computer, independantly of iTunes.
Take a look at the following breakdown of devices on cnet.com
http://reviews.cnet.com/4321-6739_7-6509002-1.html
Another new alternative is the Apple TV, which is coming out soon. For now people aren't quite sure what to expect until it is out.
http://www.apple.com/appletv/
So, to reiterate, choose the receiver that best matches your usage and pricepoint without taking in to account mp3 music streaming, and then add that later. That way you won't be limiting your search to 2-3 receivers but can consider many more that will more than adequately do the job for you.