Not sure how you want this to be setup, but if it's for home use then I'd suggest picking up an
Airport Express and setting up iTunes to play to the speakers. I had that setup for a couple years and it never failed me. Now I don't even bother since my MBP has acceptable (to me) quality sound/volume from the built in speakers.
I know the iPhone can't stream to them, but if your library is already on your computer I don't think it should be a problem.
If you're looking for a slightly more portable setup, CNET has a fairly decent
guide to getting things connected. I have a friend with a Blueant speaker in his garage that he streams to from his PC and it sounds pretty good. But for his setup I think the Airport Express and a simple pair of speakers would be less expensive and easier to manage.