I'll let others with more digital experience advise you on the sound card, but you are going to need a lot more RAM.
Baba Rumraisin said:The 700 mhz is marginal but it'll do. Depends what you want to do and how fast you want to do it.
The 20 gig drive won't last long but that's not really a problem either, just put in a bigger one as a slave.
As Lester B. said 64 megs of ram just is not enough. Multiplying that by ten would be more like it. The good thing is that its an older computer so the ram isn't going to be priced very high.
Soundcard. You could look at the Creative Audigy 2 ZS Platinum. It has a bay mounted IO. Should cost you 250 or thereabouts, maybe less. They're also the easiest to find in stores.