Hi, and welcome to Apple Discussions.
You are never going to get g speeds out of your iBook. If you are talking about internet access, 802.11b is likely as speedy as you will get unless your internet connection is above 5 megs. 802.11g is only superior when your ISP's speed tops that. 802.11g is also superior on your private WLAN. Other than that, 802.11b is fine.
From Wikipedia:
http://en.wikipedia.org/wiki/802.11b
+Due to the CSMA/CA protocol overhead, in practice the maximum 802.11b throughput that an application can achieve is about 5.9 Mbit/s using TCP and 7.1 Mbit/s using UDP.+
This is usually sufficient for internet access purposes. In fact, with my 5 MB connection, I don't notice a lot of difference between the iBook G3 800 MHz with its original AirPort (802.11b) card and the iBook G4 1.2 GHz with its AirPort Extreme (802.11g) card.
A Linksys comparison chart:
http://eshop.macsales.com/images/Items/linksys/wireless_comparison.jpg