Generally speaking, you will never notice the increased speed of 802.11g over 802.11b with anything but huge file transfers. Even then, both computers would need the new g standard cards.
The older b standard is way faster than anyone's internet connection so you will never come close to hitting AirPort's limits. I have both AirPort Extreme Macs and older original AirPort Macs functioning over an AirPort Extreme Base Station. I can't tell the difference in speed at all.
What I do like about the AE Base Station is the ability to plug my printer into the USB port on the Base Station itself and print to it from any Mac in the house, anywhere in the house. Another benefit of the newer Base Stations is the ability to link several of them together and extend the range of your network, useful in a big house or one with lots of obstructions to the AirPort siganl. You can't use this repeater effect with the older ones. Third, the new WPA encryption that's stronger than the older WEP is available only for the new AirPort Extreme network.
There are advantages to the new standard, but speed, at least for most of us, is not one of them.