I've got a 3g USB modem.

I've got a home network.

The home network includes wired LAN and wireless, using a Linksys WRT54G as the gateway and router.   There's two desktops, a couple laptops, a Wii, and a few handheld devices using the network.    Currently, the internet in connection on the router is connected to a cable modem.  I'm thinking of telling the cable company to go eat a dick.   Although I have a lot of devices, most of my surfing occurs while I'm at the office, and I'm not consuming a lot of video and no torrents etc.   I've been tracking my usage and I burn about 4.6 gigabytes of bandwidth (up and down combined) from the home connection a month.   My 3g plan is 5 gigs and only $0.03 per megabyte thereafter.

Hence my query:

I've also a spare laptop.   I want to have that laptop set up with the 3g card.   I want to bridge the 3g card to the laptop's ethernet card.   I want to take that ethernet port to the "internet in" port of the Linksys so that all the other machines on the network that look to that router as gateway will have internet connectivity.  

Is this possible?  

The laptop that will have the USB 3g modem can run XP or any recent flavor of Linux.   So how do I go about configuring it so that I can get it to do what I want?

Teach me like I'm an idiot.  Thanks in advance.

It will definitely work, but it depends on what USB modem it is.  I have one from Sprint that specifically forbids, aka auto disconnects, having other network connections open when it's connected, but my older PCIMCIA 3G modem will share the connection fine.  

Plug the Ethernet port from the laptop into the router and share the 3G connection with it.  It 'should' just work.

Anyone have some more concrete guidance?

Specifically:

Once I bridge the networks, what IP address will apply to the ethernet connection?   How do I get Windows XP (or linux as the case may be) on the laptop to connect properly to the internet port on the Router, which I assume is going to be sitting there waiting to be assigned an IP?

In response to the question above, this 3g card and provider does allow you to remain connected when there's two live network connections so bridging should not be a problem, but before I fuxor the laptop I want someone to walk me through the process.