Well, a company I would definitely go with if you want to spend money is Bose. They make some of the best speakers in the industry. (
www.bose.com)
Go with one of their systems. I have a Bose Companion system connected to my computer, and its great. (costs about 250 bucks for 2 satellites and a woofer, not bad considering I got it for free)
We used to have Bose 901's in our old house and those were awesome speakers (2 channel only but it sounds like surround). (Those are expensive buggers though, its at least $900 for 2 speakers, and each of the speakers must weigh 30+ lbs)
They arent exactly 'blow your next door neighbors out' speakers from Bose but they have some of the most life like reproduction I've ever seen.
You might be able to buy just the woofer but you might need their special EQ to connect it to your system.
Also, they use old tech connections (only analog), which is fine for me (my soundcard has no digital out) but if that's something you want, then forget about Bose. (Well maybe a few of their systems might have digital in, but mine sure doesnt)
That's my speaker experiences. I'm sure there's other speakers out there that are great too.