The part numbers for AirPort Extreme Base Stations:
January 2007 model (with Fast Ethernet): MA073X/A (now superseded)
August 2007 model (with Gigabit Ethernet): MA053X/A
In a pragmatic everyday sense, what is the significance of this hardware change?
Between the January 2007 release and the August 2007 release, ethernet speed. If you use AirPort Extreme Base Station as your router and have computers that support Gigabit Ethernet, you will see significant speed improvement when file transferring, as well as running other transfer intensive tasks such as Remote Desktop.