I have a pretty similar setup as you, except for me still waiting for my 17PB. What I'm going to do is buying a simple Airport extreme station w/o modem and it's pretty much it. You connect your PC to the LAN-port and the iBook wirelessly. The airport station is connected to the cablemodem.
Oh, I thought a bit wireless =)
If you dont have wireless in the ibook, you will need some kind of gateway. You may still use airport station, but then you'd need a hub or a switch too. You can buy some simple gateway w 4-port switch at any comp-store now, it would be enough for your needs.
Alternatively you can just buy a second network card for your P3 and set it up as a gateway. That would make a limitation in that you would need your P3 running to have access to internet on you iBook.
hope it helped
/ filipp