For the SB4200E you need the CDCEther module (and one of uhci, usb-suhci or usb-ohci already 'insmod'ed to provide access to your USB bus). If compiled separately to the kernel, do 'modprobe CDCEther' and if you're doing 'tail -f /var/log/syslog' or 'tail -f /var/log/messages' (depending on your system, I'm using Debian) then you should see that an interface is created called 'ethX' where 'X' is the next free number in your system (on my system it's eth0, so I'll continue like that).
Now, I've successfully used pump, dhclient or a late version of udhcpc to register my PC with the modem. e.g. 'dhclient eth0'.
When the modem is not connected to the internet (because you've pressed the blue button on the top and the Standby light it orange) the modem should, if its dhcp was left enabled, give your PC an IP address between 192.168.100.11 and 192.168.100.42. If the modem is connected to the Internet i.e. not in standby, the modem should negotiate with the ISP's DHCP server to obtain a public IP address to give your PC.
Either way, you should be able to do 'ping 192.168.100.1' and also view the web interface on http://192.168.100.1/.
If this isn't working, it can be because some ISPs configure the modem to only talk to one PC (actually MAC address) at a time. How to reset this may depend on your ISP.:
1. In one setup I've encountered, it was sufficient to restart or power-cycle the modem. The modem will pick up the first new MAC address it identifies and lock to that and will then talk to that interface.
2. In another setup the same applies except that you can only access the ISP's special web interface (for formally assigning the MAC address you wish to use) until you have indeed formally assigned the MAC address you wish to use. If this interface exists and you don't know about it, you need to speak to your ISP. If you have found that you can connect different ethernet cards to your modem without problem then this scenario doesn't apply.
If I have more info I'll post it at http://praguespringpeople.org/Struan/Software/Configuration/Motorola-SB4200E/