Not wanting to get too specific here, but I've recently built a couple of Integrated Duron boxes for a total of $338. No doubt I could have done better had I scrounged, used a little bit lower quality parts, etc
AMD Duron 750 BB ($200 on special- included 52X CD-rom and floppy, Chrome orb, Biostar MVKL integrated board)
Crucial 128 Memory ($40- note it's cheaper now!)
WD 7200 20 gb drive ($75- was on special at CompUSA)
Modem, Keyboard, mouse, etc (Modem $11, Keyboard & Mouse $14)
Already had couple copies of Win98 se
I'd look for either a "old" 810/815 Celery, or a integrated Duron for CPU/Mainboard. Memory is going to be about the same in each case. As far as a HD, going to be hard to find anything good IMO under 10 gig. If you go used, you may well be buying somebody elses problem! You may well want to look at some 5400 rpm drives- they sell at a much lower price than the 7200's.
I can't imagine not needing a CD, but ok, don't get one! As far as sound, etc most of the low end integrated include it, nothing says you have to hook it up to speakers