You can use any standard 802.11B or 802.11G AP/Router for wireless internet.
Things can get a little complex with WEP or WPA, but it will work.
But on the client side (PowerBook, iBook, what have you), you need the apple branded card.
Well, you can use a 3rd party PCMCIA card, but I think this is beside your question.
Yes...any 802.11b/g wireless access point will work (using a netgear myself) and as for airport cards you do either need to use Apple's airport card in an ibook or 12" powerbook, you can use a PCMCIA version in the 15/17" powerbooks or a USB dongel version on any of them as long as they have Mac drivers