And if I
wanted to add a SCSI card and/or USB card, will any old
 random PC PCI one just work, or do I need something exotic and special
 fabricated out of unobtainium? 
 USB on a Sun box I have no experience with, but ISTR there is some
 favored model of Adaptec PCI SCSI card that should just drop in - the
 unobtanium part *might* be that you'd have to find a SPARC boot ROM if
 you wanted to boot from an attached device.  I have only ever booted
 from "internal" disks on that variety of hardware.
 If you do find one with a SPARC ROM on it, it should be a matter of
 stuffing it in a slot and powering on. 
 
Pretty much any Symbios 22801 or compatible should work out of the box.
Peace...  Sridhar