Bad news: it looks like the Belkin card was a mistake, which possibly serves me right for not doing my research properly. The F5D6020, which is on the list of supported cards, apparently comes in (at least) three versions because the manufacturer couldn't be bothered to change the product number when they change the product. The version on the list is v1; it's apparently possible to get open-source support for v2 although it's not included in the distributed pcmcia-cs package, and what I've got is v3 which appears to be supported by a closed-source driver which has been compiled for a couple of 2.4.x kernels, and in any case it's a CardBus card which will not even go into my laptop. It should work on the Windows machine, but then the 3Com card was originally for the Windows machine (and the 3Com is 54Mbit while Belkin is 11Mbit). The 3Com card has a better chance of working under Linux when I upgrade my laptop (assuming I go to something modern with a 2.6.x kernel), but not until then.
So it looks like I am still wired until I get a new card and/or a new laptop. I could… hey, let's turn this into a poll!
What should 
imc do about the network card?
Keep it for use on the hypothetical new laptop.
0(0.0%)
Use it on the Windows machine and keep the 3Com card for use on the hypothetical new laptop.
0(0.0%)
Ask the vendor if I can send it back for a refund (which does seem to be possible, though there will of course be postage costs).
2(25.0%)
Attempt to flog it on ox.for-sale.
0(0.0%)
Then buy another 3Com for use on the hypothetical new laptop.
0(0.0%)
Or try to buy a version 1 F5D6020 card on eBay for use on the current laptop as well as the hypothetical new laptop (it probably won't be any cheaper than the card I've got).
0(0.0%)
Buy a new laptop.
1(12.5%)
Tickybox [did I leave anything out?].
2(25.0%)
