Hi Joe, yes that is about it. The Wireless USB adaptor and the PCMCIA card will talk to each other and you will be able to share the connection and files over the network.
However, one point is that you will need to have the desktop PC turned on all the time if you want to access the connection (Broadband) from the laptop as this is acting as a server.
Bit more expensive but have you thought of Wireless? (Which is what I am doing at present) and also, consider whether to go for 11Mbps or 22Mbps or the latest 54Mbps (Which is not that much more expensive than 11 or 22).
Mail me as I have quite a bit of info I can send you!!