This is primarily a post to myself for future Googling. It may help someone in the future, but it's not intended to be a fascinating read.
I talked Lee into getting an Airport Express because he needs to get his Xbox 360 on the internet. This was a bad move. It turns out an Airport Express doesn't bridge from wireless -> wired without being in WDS mode and Lee's router is too old to support WDS. While we were poking at it (over the phone - so I was at home) I decided to use the profiles feature on the Airport Express to try to configure mine like his. This was a worse move. I managed to put the AE in a mode where it was impossible to connect to it - requiring the old "Factory reset" trick - which trashed my saved profile. Grumble.
After some Googling about Lee and I were both convinced he needed to take his Airport Express back and get a Netgear or Linksys product. Now all I had to do was put mine back together. That turned out to be complicated and non-intuitive. If you do things wrong the Airport Express simply boots up into an unreachable alternate universe, forcing a hard reset of everything.
So, here's the working Jesternet configuration. DSL line comes into the DSL box. DSL box plugs into WAN port on the Airport Extreme (802.11g base station). Main switch plugs into LAN port on the Airport Extreme. Bunch of crap goes into the switch. At the other end of the house the Airport Express streams music to the family room and serves as a wireless repeater.
Settings for the Airport Extreme Base Station:
Airport tab: Name is set to Jesternet Base. Be sure to set the password for the box itself (via the Change Password button) Under Base station Options everything is cleared for "WAN Ethernet Port" Under AirPort Network the Name is set, Create a closed network is NOT checked. Click Change Wireless Security and set the security to WEP (can't use WPA with the Express or the Nintendo DS), enter a password. Set the encryption type to 128 bit WEP. The Channel is set to 10 and the Mode is 802.11b/g compatible (DS and PSP both need b) Under Wireless Options Multicast Rate is set to 1, and the Enable interference robustness flag is cleared.
Internet tab: Connect Using is Ethernet. TCP/IP is configured manually - with stuff given from the ISP.
Network tab: Distribute IP addresses is checked. Airport Client computers share a single IP address (using DHCP and NAT) the address range is set to 172.16.1.1Everything else is cleared.
Port Mappings tab is set accordingly - I'll document those somewhere lesss public thanks :-)
Access Control tab is blank.
WDS tab: Enable this base station as a WDS main base station. Allow wireless clients on this base station is checked. The MAC address for the Airport Express is listed
For the Airport Express
Airport tab: Name is set to Family Room. Set password and Base Station Options just like the Extreme. Under the Airport Network select "Create a Wireless Network (Home Router)" - THIS IS THE KEY BIT. Even though this is a WDS client of the Base Station it is NOT set to be a client here. If you do it will go conjure up a magical IP address that isn't reachable from the LAN and need a hard reset. Set the name to Jesternet and set all the other options to match the Extreme - except the channel is set to 10 and the Multicast is set to 2.UPDATE 3/1/07 The channel should not be set to 11, it should be set to 10 (same as the base station) Music streaming doesn't work right otherwise.
Internet tab: Connect using AirPort(WDS). Configure using DHCP. Also allow wireless client computers is checked.
Network tab: Distribute IP addresses is cleared. This greys out everything else.
Port Mapping tab: Blank
Access Control tab: Blank
WDS tab: Enable this base station as a WDS remote base station. Allow wireless clients on this base station is checked. Click the "select" button and browse for Jesternet Base.
Music tab: Enable AirTunes on this base station is checked. iTunes Speaker Name is set to "Family Room".
Whew! Never mess this up again.
Not the most interesting point I realize, but an important historical archive :-)