PCMCIA works fine most of the time, I'm using it daily with a WLAN card and I've also tried it using regular network card. The kernel module yenta_socket and the pcmcia-cs package (or in experimental kernels, the hotplug package) should do the trick.

There is one gotcha though: PCMCIA fails to function if you have 1GB of RAM and use an older Linux kernel. The problem has been fixed in version 2.6.11 (see this thread for details), so an update is strongly recommended (I haven't checked the 2.4 series).

Should you have an older 2.6 kernel, and no wish to update, or a 2.4 kernel you can try either the following patch which was found by Vincenzo Belloli <vicky@belloli.it> in this thread, or this patch as suggested by Peng Li <lipeng@cis.upenn.edu>.