Charles, you can have my old Nforce2 mobo (A7n8x, I think), Socket 7 CPU (probably a Barton at 2 GHz or so), and ?MB of memory (likely about a gig), case, and PS for shipping and beer money. It's got a gob of free PCI slots.
My Canon FS4000 scanner runs dandy on a low end Trantor SCSI/PCI card. There's no need to get some ultra/mega/150/SCSI7x card. A fast hard drive will help quite a bit if your computer has FARE or ICE hardware IR dust detection.
Slide scanning is hugely time intensive. Unless you have one of those fancy auto-feeders, you'll be lucky to finish more than a box or two per evening. I suggest you buy a nice light table and use it to ruthlessly crop the lesser images from your scanning queue.