On the Powerbook, open the Sharing system preference, and enable Windows sharing. That will allow the Powerbook to server files to the Windows machines. Then, to connect to another machine, just use the network icon in the Finder, or "Connect To" under the Go menu.
The Linksys WRT54G should do just fine. If wireless signal strength ever becomes a big issue somewhere in the house, three options exist:
1. Use 3rd party firmware on the Linksys that allows you to boost the power. Sveasoft is one version that adds more features and allows you to set the power higher.
2. Buy a WDS compliant wireless extender, like the Airport Express. This can extend the wireless range on the Linksys, and also serves 2 other functions. It can share a USB printer to both systems, and can also have music sent to it if you attach speakers. One other advantage if you travel to hotels with broadband. The Airport Express is a smidge larger then the Powerbook power supply. So just throw it in the laptop bag and plug it in at the hotel.
3. Buy a WAP54g or other base station (the Airport Express would still work as well) and run a CAT5 cable through the house from the WRT54g. If you set both base stations to the same wireless network name and encryption, your computer will pick the one with the strongest signal and roam between them without dropping the connection. This works better then WDS, as it has no speed reduction due to the relaying taking wireless overhead. Just make sure to use different channels on each base station, and seperate the channels by at least 5.
Oh, out of the box, make sure you change the channel. In the US you get channels 1-11. Most equipment defaults to 6, and neighboors with wireless likely didn't change their settings. My two access points run at 1 and 11, as those are the only 2 US channels with no overlap on the frequencies 6 uses.