Let me just ask why 11mbps is too slow for her...

Keep in mind most broadband internet (like a fast cable) would be in the 1-4 mbps range. Therefore meaning your wife would see absolutely NO difference going from 802.11b to 802.11g speedwise if you had internet slower than 11mbps (pretty much any household user).

She might be able to eek out a little more distance with her powerbook if she has an 802.11g card, but at far distances the speed drops substantially.

If she has an Aluminium powerbook, then she is good to go and can buy an airport extreme card. But i'm guessing she probably has a titanium one, and that wouldn't support apple's extreme card.

Therefore you are stuck with buying a USB dongle (which I wouldn't recommend) or buying a PCMCIA card (which I would recommend!).