Hello,

  GM> We're using an old 486/33 box for Samba, but it's pretty slow.

That's probably to be expected.

  GM> It seems to me the bottleneck is the NIC.  (All other devices on
    > the network have 10/100 interfaces.)

As John explains in more detail, the whole machine is probably the
bottleneck.  Even a top-end 133 MHz 486 (or 5x86) with a PCI bus will
lack the throughput for anything but trivial file-serving needs.  The
efficiency of the software and protocols involved may also contribute
to the perceived performance, but it sounds as though you're not
getting the throughput you want.

  JN> Here's the catch.  The ISA bus tops out at about 4 MBps, or
    > about 40 Mbps.

ISA's limitations are significant, and one of the reasons that I tend
10baseT cards to 486 machines (that have ISA or VLB).  I reckon the  
absolute theoretical maximum throughput of an 8MHz, ISA bus is 8
megabytes per second for an 8-bit slot and 16 megabytes per second for
16 bit slots at the same clock.  It's possible to crank up the ISA
clock on many machines, but not all cards will cope.  In any case, the
real-world throughput is likely to be less than my figures.

  JN> Dump it and use a real machine.

Perhaps dump it as a file server.  486 class machines are still
capable of useful work in 'light duty' applications.

Regards,
  - Andy Ball.