2wire - Motorola Surfboard SB6121 modem conected to 2WIREi38HG wireless router but there's no internet access

My home network was using a linksys WRT54GL in the "traditional" - WAN port is hooked to the DSL modem, once of the ports on the linksys goes to another switch and most of the computers in the house are wired to that 2nd switch. Everything was using the default 192.169.1.x subnet, so it looks like:

                    Wireless Devices
                           |
 [Modem] <-> [Linksys]  <->      [Switch]
                            |                          |
                     Computer1         Computers

Last week, I upgraded my internet connection, and the modem is now included in the router - it's a 2wire, which also does DHCP, etc. that uses 192.168.2.x. The network now looks like:

                                                                                      Wireless Devices (x.x.1.x)
                                                                                                    |
 [2Wire x.x.2.1] <-> [Switch]  <->   [Linksys - WAN=x.x.2.2, LAN=x.x.1.x ???]
                                       |                                                             |
                          Computers (x.x.2.x)                                Computer1 (x.x.1.x)

2Wire is now the DHCP server, and DHCP on the linksys is disabled.

Is it possible to continue to use 192.168.1.x for Computer1 and the wireless devices somehow? I tried connecting the WAN port of the linksys to the switch, and setting the router IP to .2.2, but that didn't seem to work.

The goal is to use the Linksys to serve DHCP to the wireless devices, and computer 1, while using DHCP from the 2wire for the rest of the computers. (The linksys allows me to specify the DNS servers, which means I can use opendns to restrict access to "safe" sites - wireless devices are ipods and Wii, and computer1 is also for the kids). 2wire doesn't seem to allow you to specify alternative DNS, it just passes on what it gets via PPPoE.

Is this possible, using the linksys firmware? Would it be possible with the DD-WRT firmware?

The Linksys and the 2Wire can do DHCP on their own LANs at the same time. Let the Linksys router receive an IP from the 2Wire, don't assign it. You don't need DD-WRT for this (although I'd use it anyway).

|--------------|                                              
|   2Wire      |              |---------------|   Wireless Devices
| WAN=(ext ip) |              |    Linksys    |   (x.x.1.???)
| LAN=x.x.2.1  |---[Switch]---| WAN=x.x.2.??? |     |
|--------------|       |      | LAN=x.x.1.1   |-----|
                       |      |---------------|     |
                       |                            |
            Computers (x.x.2.???)                 Computer1
                                                  (x.x.1.???)

In this setup, both the 2Wire and Linksys have DHCP on. The "???" means it receives a dynamic assignment from its DHCP server: Linksys WAN and Computers get it from 2Wire LAN, Wireless Devices and Computer1 gets it from Linksys LAN. You could pretty much just reset the Linksys to its defaults, then configure it with a LAN address of x.x.1.1 and plug it in. LAN DHCP is on by default. WAN will receive a dynamic IP by default (from 2Wire).

I use 2 routers in my house, A cheapie Trendnet and a Netgear N

The netgear N has the DHCP server, the trendnet has that disabled.

             Wired Computers
              |
Modem----Netgear N------Crossover cable----Trendnet G---Wired computers
             |                                 |
             |                                 |
             |                                 |
Main Wireless Network                       other wireless network.

That's possible, and I'd suggest DD-WRT/OpenWRT/X-WRT for the Linksys for extra configurability. I think you'll need it.

If the 2Wire is doing its DHCP properly, and serving out the x.x.2.x network, it doesn't really matter what IP the WAN interface of the Linksys gets -- unless you want the x.x.2.x machines to be able to talk to the x.x.1.x machines.

If that's the case, you'll need to see if the 2Wire will let you set a static IP -- maybe MAC-based via DHCP -- so the Linksys always gets the same IP on its WAN port (then you'll want it to be x.x.2.2). You'll also need to set host routes on the x.x.2.x machines to tell them that x.x.2.2 is the gateway for the x.x.1.0 network. (You may not need host routes if the 2Wire will let you configure a route so that it forwards packets to the x.x.1.0 network to x.x.2.2 -- then it does the internal routing for you.)

It's possible the 2Wire just isn't configurable enough to do what you want to do, though. In that case, there's another option.

Now with VLANs!

One more possibility you should consider is using your traditional WAN wiring (your first diagram, 2Wire -> Linksys -> devices/computers), but utilize VLANs in DD-WRT to do the network segmenting for you.

Go back to your original wire diagram, but this time configure VLAN1 to be (whatever port your switch is plugged into) and VLAN2 to be (other ports + wireless). DHCP requests on VLAN1 get one set of network settings -- x.x.2.x network, standard DNS settings, whatever; requests on VLAN2 get the other set -- x.x.1.x, OpenDNS filtering. The Linksys can do all the routing internally for cross-segment traffic, and naturally any internet-bound traffic gets routed out the WAN port.

This is a bit of a pain to set up, and some of it may not be supported by the configuration webGUI that DD-WRT/X-WRT offer, but it is all possible internally.

Edit: "Internally" means "via the ssh interface" -- ie, commandline control of DD-WRT.