At my end of the world (New Zealand) we use ADSL with PPPoA, but for what it's worth, this is how I generally set things up.
Let the DSL modem/router look after the DHCP and NAT (if required) and PPPoA (or PPPoE). Set up the AEBS in Bridge mode using either DHCP for its address, which it will get from the DSL/router, or with a safe address in the same subnet as the router but outside the range that any other devices on the LAN will be allocated by DHCP.
In my case, my DSL router is 192.168.1.254, and I have set it to distribute (by DHCP) the addresses 192.168.1.1 - 192.168.1.200. That way I can set servers and anything else that needs a fixed address in the range 192.168.1.201 - 192.168.1.253.
I like network devices like AEBS's to have fixed addresses, so my three Airport Base Stations (I am in a four story apartment with concrete floors) are 192.168.1.250, 192.168.1.251 and 192.168.1.252. I know that the AEBS's could be allocated addresses from the main router by DHCP, but I happen to prefer it my way.
Hope this helps.
