The evidence really speaks for itself:

firstly, I have been running a 2.2.18 kernel system, with a 3c509b and a 
soundblaster 16 (and sundry other hardware).

The soundblaster 16 is on 0x220, irq 5. Its a soundblaster 16 (vibra 16b, '94)
The 3c509 is pnp and detects under 2.2.18 as the following:
eth0: 3c509 at 0x300 tag 1, 10baseT port, address  00 a0 24 75 b7 28, IRQ 10.

Both cards work perfectly, and autodetect without any arguments.

Now:

Here are the interesting bits of the boot of the 2.4.x kernel:

PCI: PCI BIOS revision 2.10 entry at 0xfb4f0,
last bus=1
PCI: Using configuration type 1
PCI: Probing PCI hardware
Unknown bridge resource 0: assuming transparent
Unknown bridge resource 1: assuming transparent
Unknown bridge resource 2: assuming transparent 
PCI: Using IRQ router ALI [10b9/1533] at 00:07.0
isapnp: Scanning for Pnp cards...
isapnp: Card '3Com 3C509B EtherLink III'
isapnp: 1 Plug & Play card detected total

eth0: 3c509 at 0x220, 10baseT port, address  00 20 24 75 b7 28, IRQ 5.
3c509.c:1.16 (2.2) 2/3/98 becker@cesdis.gsfc.nasa.gov.

Soundblaster audio driver Copyright (C) by Hannu Savolainen 1993-1996
sb: No ISAPnP cards found, trying standard ones... 
sb: I/O, IRQ, and DMA are mandatory

NB: PCI stuff was interesting, but I'm not sure if its connected to this
situation.

After bootup, at a random time interval between 10 seconds and 5 minutes the
following error spams the screen:
eth0: infinite loop in interrupt, status 2001

I can only conclude that the kernel has mistaken an ethernet card for a
sound card.

for convience, here is an lspci:
00:00.0 Host bridge: Acer Laboratories Inc. [ALi] M1541 (rev 04)
00:01.0 PCI bridge: Acer Laboratories Inc. [ALi] M5243 (rev 04)
00:07.0 ISA bridge: Acer Laboratories Inc. [ALi] M1533 PCI to ISA Bridge
[Aladdin IV] (rev c3)
00:08.0 VGA compatible controller: Matrox Graphics, Inc. MGA 2064W [Millennium]
(rev 01)
00:0f.0 IDE interface: Acer Laboratories Inc. [ALi] M5229 IDE (rev c2)

Stephen Thorne