Is there a way to configure Mac OS X Snow Leopard server as an Internet gateway with bandwidth tracking per user?
There's a RADIUS server in OS X Server, and it can easily be used for Wi-Fi authentication.

However, traffic accounting is a different thing here. Mac OS X uses Freeradius, which can collect this information, but it is unconfigurable through Server Admin. So, you'll have to set up this by editing proper configuration files manually.