Wireless Music Playing

So I'm trying to figure out a good way to stream music around my apartment. There are some options out there, but most are proprietary methods, including Apple's AirPort Express.

My desired goals are this:
1. Playing any music, regardless of source (Last.fm or iTunes usually), should be playable in my bedroom, living room, and bathroom.
2. The selection of which speaker is online would be my choice -- even better if I could setup something like an IR switch to turn on speakers in any room that I walk into or am in.
3. Preferably it can play across any OS I'm running on the primary PC -- so one would hope it could play music regardless if I'm in Windows or Linux.

Right now, the AirPort Express can do some of this, but it's limited to working with iTunes, so Last.fm wouldn't work, and it's also $99 for each AP -- somewhat overkill in that I'd need 3 (plus the two wi-fi routers I already have) to cover each room.

I'd like to find or build a better solution -- perhaps something Linux based, although if I were to try to build it, it would likely be much more expensive than just purchasing two or three Apple AirPort devices.