>ISTR reading somewhere that that's the general pattern:  Multiple similar 
>PCI cards will generally work fine, but with ISA cards, it's going to be 
>a pain (if it's even possible).

Yes, it's possible.  You mentioned using Dlink DE220 ISA's, I've built
this identical router before.  There is a floppy disk that comes with the
card (you can also download it) that writes the IRQ/IO stuff into the
firmware.  It's kinda the same as using jumpers, but it uses software to
flash the firmware instead.  You need a DOS bootable disk to do this.

Assign the cards some free pairs of IRQ/IO's (I usually use 10/300 and
5/220 because they're easy to remember) and set them up in
/etc/conf.modules.  Do an 'insmod ne2000.o irq=10,5 io=0x300,0x220' and
voila, eth0 and eth1 appear. Two ifconfig's later and you have a
router/firewall.

Just remember to label the cards so you can tell which is your inside
interface and which is outside :-)

-Brian