I couldn't live without a sound card on my PC - and while I admit that I have a bit of a Holy Grail mentality to pursuing more realistic sounds, I still have the Creative Labs Sound Blaster 16 that I bought last year for about $AUD 110. I have since bought cheap clones from PC recyclers for $10(ish) for other people to share music, and they appear to work just as well. You can pick up second hand real SB16's for about $50 through the Trading Post or similar classifieds.
I am thinking of investing $300 (Aus) on a Yamaha daughter board to dramatically upgrade the sound realism, but if you are more interested in the music than the sound, then the up front cost can be very modest.
My PC is a 486 running Win 3.1, not exactly state of the art, and I enjoy immensely picking various kinds of music from the Web. Apart from the tunes on the DT (which strictly don't need a sound card) there are vast riches of other folkie stuff, especially Celtic. I don't know about compatibility with previous generations of PC's.
The real trick seems to be picking up and installing the sundry sound drivers. I would hope that a number of Mudcat readers, including myself, could help in this regard.