I have a couple of "minimalist" PC's running Seti--a BP6 with dual celeries, 64MB RAM, an old vid card, NIC, and a HD, and a BH6, Celery, 32 MB RAM, old vid NIC and a HD. I'm using two old power supplies and a small wooden rack with pegboard shelves, sits on the floor under my desk.
I temporarily put the hard drive in a working PC, transferred OS files, then connected to the MB of the rack, switched the monitor cable, plugged in a KB and installed the OS. Once networked, install and run SetiDriver and Seti Spy so you can monitor across your network.
This place
Liquidation has some $15 540 MB HD's which may be enough to install W98. Hunt around and you should be able to find some $10-15 power supllies.
EDIT: Just remembered that Networkman was selling some stuff here, including some very cheap vid cards ($5-7?). It seems like Russ or someone had a line a while ago on cheap PS's, perhaps they'll post. Cheap RAM is all over right now. I'd stay away from a case personally, makes cooling more of an issue and you probably won't be able to disable the PS fan which adds to noise.