I am not the best person to answer some if not all of these questions. With that out of the way, you by no means need a high end receiver to get GREAT sound. Just my opinion, but electronics are so cheap and advanced now-a-days, you really get decent quality from medium to cheaper items. You should still do some component research to find out what are the best items for the price. I bought a cheap 5.1 speaker setup of 6 years ago. I can't remember the price, but it was a no-name brand for I think $300. It's been great and still is. You could definitely get better sound, then again, it's miles above PC speakers like you said and it sounds great with movies and music. I'm a big fan of digital, but I think most everything supports digital sound now. Don't by any Monster cables. You can get perfect cables supper cheap off the web. That's one type of item you can almost never get a decent deal on in a store, but the Monster cables, which are popular for some reason, have been shown repeatedly to be undetectable as far as quality goes in blind taste tests. And they taste awful, too.
I'm not sure about the sound card. I'm trying out the one on my moboard. You gotta make sure it's Ubuntu compatible, for sure.
Oh no, blue tooth sound is digital and perfect. I use it in my car. It's undetectable in difference from direct sound, unlike broadcast or radio transmitter setups.
Not sure I understand the full scope of the streamed music question. Do you mean off the Internet? That comes in lots of flavors. I think some places give free lower quality streams, and better ones for pay. I've been using Pandora (free) on my iPhone and it's around analog FM radio quality. Been meaning to check and see if a paid membership will get my higher quality. Any high speed internet has the bandwidth for perfect HD music, it's just a matter if whether the delivering site supports it. I just realized I hadn't noticed any reference to Internet streaming music in Lmce. That'd be a good feature. Wonder why it's not there? That being said, you probably just meant streaming your own music. The stream will be the quality of your files. CD will be top quality. If you rip to Ogg or Mp3, I suggest a minimum of 192kbps. I like 256, it's slightly over what I can detect.
In summary, SonicWind says: 256kbps (MP3 format), as many channels as you can, but probably stereo source is all you will have? digital outputs->yeah!, bluetooth->yeah!
I thinks someone else could give you more detailed input on specific sound cards and speaker brands and setups since my ears are kind of slutty.