Firstly, if you can afford it, go for the "new" standard of 802.11g (Not "B") it is twice as fast as the 802.11b standard and should be certified by WIFI this summer. Most major manufacturers are already shipping these products.
Right, as to your set up.
If you get the Wireless router, make sure that it has a "normal" wired output connection as well as offering wireless networking - most do.
You would set this up and connect to your PC with Cat 5 cable and so the desktop would be "Hardwired" onto the network. This will also mean you need a network card for the PC which cost as little as £10 for a 10/100. you may also need a USB - RJ11 "Dongle" to convert the USB output from the ISDN to the Network card.
It is then a simple job of installing the Wireless Cardbus software onto the laptop and then running the Network Setup Wizzard in XP on all the machines (Starting with the Desktop).
Away you go! In all it takes about ten - fifeteen mins to complete.