Whether you at an Airport card to your PowerBook depends on what you want to do with it. The Airport card will need an Airport Base Station or other Wireless Access Point to talk to. Unless your Windows desktop also has a wireless card, I don't see what the Airport card will add to your current configuration without additional hardware.

It's possible for a Mac to share an Internet connection with another computer connected to it (though with everything is wired via Ethernet, the Mac would need a second Ethernet port installed.) I don't really recommend this.

You can share a high-speed broadband Internet connection between more than one computer using a broadband router. These have an Ethernet cable connection for connecting to the Internet (WAN port) and multiple Ethernet cable connections for connecting local computers (LAN ports). A wireless broadband router will also allow an Airport-equipped PowerBook to connect to it, if you want to go wireless with your laptop.

These routers are so cheap these days, it's really not worth going through gyrations to establish the Mac as a sharing device.

Some Wireless (WiFi) router examples: LinkSys, DLink, NetGear. Belkin, SMC, etc are all equally good, too.

Those are 802.11b wireless routers, which is what Apple calls "Airport". A higher-speed, backward-compatible 802.11g wireless access (what Apple calls "Airport Extreme") will give you higher speed, if you have a newer PowerBook that can support the Airport Extreme card.

MacWorld articles on home networks