Apple - Support - Downloads - AirPort Utility 5.3.1 for Leopard. Wow, incredibly hard to find, but the Airport Utility let's you look at individual client performance. Very cool, but impossible to find but it shows you the nominal bit rate of the link I think that is active and changes over time.
here is the sequence:
Start /Utilities/Airport Utility
Click on the left hand side on the Airport or Time Capsule you want
Click on Manual Setup
Click on the Advanced Button at the upper right
Click on the Logging &SNMP tab
Click on the Logging and Statistics button
Click on Wireless Clients
And I thought Microsoft was obscure!