IIRC, the 802.11b cards from the PowerBooks were the same as the ones in the iMacs, and the 802.11g cards from the later Powerbooks were also the same as the ones in the later iMacs.
However, the 802.11b and 802.11g cards are different form factors from each other.
802.11b:
802.11g:
Only the 802.11g version is really recommended, since the 802.11b cards don't support WPA2. Unfortunately, that really limits your choices for the iMac.
Specs:
http://en.wikipedia.org/wiki/IMac_G4
I have a 1.25 GHz 20" iMac with USB 2 as well. The USB 2 is nice, but truthfully it's so slow it doesn't get used much either. It's too slow to run anything Flash related these days, so effectively it's just a casual surfer and email-class machine. Actually, I don't even use it for that. I just use it at work store and display images, but I might just stop doing that too and instead just store everything on a 64 GB flash drive. (At the time I brought my iMac to work, my work computer was an old crappy low-capacity Win 2000 machine, but it was replaced with an XP machine several years ago.)
Like with my 15" iMac G4 800, I think I'll just downgrade it to 10.4.11 and stick it in a guest room or something.