The output quality has a lot to do with the size and type of plasma display used. An EDTV, regardless of size, will typically give you 852x480, probably worse than what you're used to for a CRT/LCD. The smallest plasmas start at around 1024x720, while larger ones get up to 1366x768 or thereabouts.
Assuming you have a video card with a matching resolution, the trick is making the connection to take advantage of it. Neither component nor S-Video are adequate, so you'll need (at least) a PC monitor or composite input; you'd probably need an adapter for the latter.
I'm not an expert on the subject, but I find it unlikely that dual-source speakers exist. For starters, you'd be dealing with a lot of wire. It might be easier to connect your PC audio out to the HT receiver.
I am trying to set up a small home theater in my bedroom where there is limited space. I was wondering if I can buy a plasma tv that can be use as my computer monitor as well and in my home theater. If so, is the output quality as good or better than if I had use a separate computer monitor(either crt or lcd).
Also is there a speaker set that I can hook up to my home theater and separate computer to output sound?