Yikes, sounds like a logistics nightmare!
Hope you don't mind drilling and running cables down inside walls! You could split the output from your computer sound card using an 1/8" stereo Y splitter, and run one to your computer speakers and the other to a long cable to your receiver downstairs as an AUX or TAPE 2 or any other spare input that can be switched in.
Many receivers have a speaker A/B switch that allows you to turn on or off two completely different sets of speakers. The A speakers would be your home system downstairs, while the B system would be wired back up to your bedroom.
I've tried some less ambitious two room setups using the speaker A/B switch on my receiver, and it had very limited use. When the stereo is in a different room than where you're listening, you have to keep going back into the other room to change the volume, or switch music track, or even pause it when the phone rings. The only use I found for the remote sound was a) during housecleaning for background noise, and b) while throwing a party for background noise.
With the price of receivers being as low as they are today, I think you'd be much better off just buying a separate inexpensive receiver to power the room ceiling speakers, and get a DVD or CD player that can play back MP3's for downstairs. Then you'll have it all, and won't need to take the trip up the stairs to change song, or the trip downstairs to turn off the stereo when you're ready to go to sleep.
