802.11g in a mixed b/g network only delivers about twice the performance of 802.11b by itself: Matthew Gast, author of 802.11 Wireless Networks: The Definitive Guide, walks through the math of how fast 802.11a, b, g, and b/g networks really are. His conclusion is that a b/g network can only run at about 1.6 to 2.4 times faster -- measured in actual data payload -- than an 802.11b only network.
The good news is that 802.11g in its pure mode does run about five times faster when measuring real data.