So how is the Westall DSL modem/router connected to either computer?

This may be different for your system, but generally speaking the setup goes like the following.

You have the phone cord coming from the wall to the DSL modem/router which interprets the DSL signal and turns it into something the computer can use. You then hook the ethernet jack from this router to the WAN port on any other router, such as the WRT54G.

In your case, I would then connect the SPA to one of the normal LAN ports on the WRT54G.

So you have a setup of cord from wall --> Westell --> WRT54G --> SPA, 2400, E510

You'll have to work with your ISP to make sure the WRT54G and Westell are communicating properly. Generally I would just recommend putting the Westell into bridged mode, and doing a PPPoE connection with the WRT54G, but this might be a bit much to tackle at the moment. First just try and get something working, write down all the settings and anything else you'd need to recreate this, and then you can start looking into setting up a bridged connection.

And on a somewhat unrelated note, it would be quite behooving of you to update both systems to SP3 if they're both going to be connected to the Internet.