SME7Admin from Landry Breuil provides several mrtg-style graphs, including a graph of "bytes in / bytes out".
I've only used it in "Server Only" mode, so I don't know if it will give you reports on each NIC in "Server Gateway" mode.
The SME7Admin Install How-To worked flawlessly on a virgin SME 7.0 Final system - although I did quickly decide to change the default "notification" limits...