Hello,
An Airport card is basically 802.11(b), and an Airport Extreme is 802.11(g). As long as the Airport can do the basic wireless, you will be alright. You might not have the latest encryption available, but it should work.
Your Airport card should work with any base station out there... the Apple base station, or the Linksys wireless routers, or the Buffalo Tech ones, or the Netgear.
My Powerbook G3 will not use an Airport card... I use an Orinoco card with a Buffalo Tech wireless router. I have WPA (I think) encryption working just fine.
What I would do is the following:
* Purchase a wireless router. Either the Apple Extreme Basestation, or a competing product, such as the Netgear, Linksys, or Buffalo Tech unit. I personally do not like Linksys due to RF interference their boxes like to generate. If you get the Apple Extreme basestation, it only has one lan port on it, so you might need to add a switch if you are going to have more than one hardwired computer.
* Use the system profiler on your laptop to see if there is an Airport card in there.
* Wire the cable modem to your WAN port on the device you purchased above.
* Wire the desktop to your LAN port, or into a hub connected to the LAN port. Be sure that the cable connecting the LAN port goes to an "uplink" port, or use a crossover cable. Some new hub/switch designs have autodetection on the uplink port, so this may be moot.
* Use the desktop on the LAN port to configure the webpage configured firewall.
* Use the desktop on the LAN port to configure the wireless setup. Be sure to setup encryption. Do not run around "naked".
If you need more deeper instructions, let us know what you decide to purchase, and we can go from there. If this discussion "dies", feel free to email me to bring my attention to it. I have a lot going on.
Christian