The easiest way to do this is to buy a home router like those from Linksys or Netgear.
 
I use a Linksys and I have it connected to my Motorola Surfboard 5120 then all the machines in the house can connect to the net using the single cable modem.
 
I can't say the dual USB/Ethernet connections will work at the same time, but maybe they will.
 
One thing to look at is to see if there is a firmware update for the surfboard as perhaps if you are having issues getting these to work at the same time, there maybe a fix for this
 
Good Luck...

You can not use the Ethernet and USB connections at the same time. Most cable ISP's only allow you one physical address (MAC) attached to the modem at a time. What you wanted to do would require the modem being able to have more then one NIC or USB MAC address.
 
Best to just buy a router as was suggested.

I have the same modem at home and for some reason it does say it is possible to do ethernet and usb at the same time, but I've contacted Motorola and they said it is NOT possible to do so. Can find a cheap Linksis/Netgear/D-Link router on the internet somewhere, and that will solve your problem.
 
try:
www.ecost.com
www.buy.com
 
they often have good deals on home routers.

Thanks guys.
 
After about 2 hours of messing around, i've come to the same conclusion; Sb5100 has no routing capabilities what so ever.
 
Seen a linkys for about 30 quid, possibly get a second hand one cheaper, me hopes.
 
Cheers for the help.