Larry,
Whether or not the decoder box can be bought seperately, I don't know but it's a moot point, you couldn't integrate it into your current setup even if you wanted to. Although digital will make a very miniscule difference in sound quality, the average person won't be able to tell the difference especially given the usually cramped quaters of a speaker setup in a PC environment.
Your current 5.1 setup is more than sufficient for DVD's, but take note that many games do not output to 5.1, only 4.1 so you won't have the centre channel functioning as intended.
My advice is to stick with what you've got unless you're looking for something with a lot more power. Check out
http://www.logitech.com/index.cfm?pa...2&languageid=1 for a really hot set of speakers. Prices are in CDN but I think they run about $300 in the US if not lower.