Here's something that might interest you, its called the Simple Fi by Motorola, and it streams MP3s from your computer wirelessly to the Simple Fi box (which plugs into your receiver via RCA Jacks). Its kindof pricey right now, but it would do what you want and allow you access to a ton of MP3s (already on your computer)...
Simple Fi
Alternatively, you could buy a $199.00 Xbox, mod it ($59) and put a 120gig HD ($120) or however big you want ... Dump a bunch of MP3s onto the HD and play them. You'd need a TV in the same room that you wanted to play your music, but it would work! Plus you could do optical out as well as RCA. Additionally you could play Divx movies and the like, not to mention you could also stream your MP3s through the network similar to Simple Fi *and* read burned DVD discs with MP3s on them (Personally, my xbox can read DVD-R/RW and DVD+R/RW)
If you just use the DVD route, its a $269 purchase (I'm assuming you'd also buy the DVD remote to control the Xbox remotely).
Not the cleanest method to do what you want, but it would work. Simple Fi might be a better option for you if you don't like messing with things to get it to work.