On some systems the "Speed" property in Win32_NetworkAdapter shows the link speed (100000000 = 100 Mbit) for the ethernet adapter. Unfortunately, on the majority of systems I've checked (mostly HP) there's no value for this property.
You may be able to add the script you mentioned as a custom action (wscript \\SERVER_NAME\SHARE_NAME\SCRIPT_NAME.vbs {computer}), but it will need some work: it needs to accept the computername as a command line argument. Even then, it will probably display rubbish if the system you're checking has more than one adapter installed (Bluetooth, Firewire, Infrared, WiFi, etc.). Try and run this script on a notebook computer, you'll see what I mean. Unless you have a lot of systems with a single Intel or Broadcom NIC, I'm afraid this will prove to be a dead end.