Bottom line wants/needs:
   - Lots of disk space, hopefully spread over several physical spindles
   - 4 Ethernets (maybe could do with one less)
   - 1 FDDI (I don't expect to be able to get an FDDI card for a NetBSD/i386 PC)
   - Console head doesn't really matter much

Except for the FDDI, this all sounds doable.  It will be a tight
squeeze for interrupt lines, though.

   - Only PCI SCSI support is for an "NCR 53C810 PCI SCSI host adapter"; how
     stable/reliable are these cards/software support in NetBSD for them?
     (The Pentium-100 PCs I've seen advertised with SCSI have had an Adaptec 2940;
      I see these are under "Hardware that we do NOT currently support".)

The BT-946C should also work.  (As long as you don't configure its
BIOS to overlap with some ethernet card's memory, that is.  Hi
you-know-who-you-are.  B-))  The kernel pretends it's an ISA card, but
it still does PCI DMA, etc.

   - How much memory can NetBSD/i386 support?  Is there any problem with >16 Mb
     configurations?

With all 32-bit DMA masters (excluding the floppy controller), the
hard limit is pretty large, except that the CMOS will only report up
to 64MB.  You may have to modify the kernel slightly to recognize >
64MB of memory (unless you want to try a (small) experiment B-)).

   - What are the best/most screaming Ethernet cards for a high-end PC that
     NetBSD supports?  I don't know anything about the WD/Novell/3Com/AT&T cards.

PCnet-32 allegedly works (with an up to date if_le.c), and would
probably be my pick.