Using a seperate router with the Pix 506e

Having been told that the 506e will not route on the same interface I need
to setup our old linksys router to do the job.

The question is then about the setup. This is our network diagram

Other Office------ modem-----router
|
|
dsl modem----pix----linksys router-----network a
|
|
dell router
|
|
network b

Other office is a secure direct dsl connection.

The dell router is setup as a gateway to Network B. If a computer on Network
A asks for an IP address in Network B I need our Linksys to send the
connection over to the dell router so it goes down there properly. I have
this all setup in the routing table of the linksys router and it works fine
when the linksys is our gateway to the internet. Now the Pix is in the
picture and it can't take on that responsibility so I need to get the two
working together.

My main confusion is on the ip addresses for the pix and the linksys. If I
have this right the Linksys will be seen as the default gateway by the rest
of the network right? So should be 192.168.168.1 on its Lan IP.
192.168.168.2 on it's Wan ip and given a default gateway of 192.168.168.3
which will be the Pix. The pix is then left with all its outside
configuration as normal.

We tried this configuration and had no joy. The Pix seems intent on changing
its IP back to 192.168.168.1 whatever I do. Any chance of some idiot proof
instructions? I normally use the PDM but we have managed to get a
permanent cable into the console access now so I can do it from there if
need be.

Also should it be a crossover cable between the Linksys and the PIX?

Thanks very much

regards

Victoria