Connecting AirPorts using an Ethernet "backbone" is by far the best way to configure a network.
See the information about a "roaming" network in various topics in this Apple support document. Ignore information about WDS since this is an old outdated system.
Unfortunately, the document is a bit out of date as far as configuring the devices to work with the Lion and Mountain Lion operating systems, but the general information about "roaming" is quite good.
To confirm, please tell us what you already have setup and working, and what you need to add.
It appears that the AirPort Extreme is already set up and it will be your "main" router, correct?. If the AirPort Express is now set up to "extend" wirelessly, then the configuration will need to be changed if you plan to connect to it with Ethernet.
We will assume that you have already tested all of the Ethernet cabling to make sure that all connections are good.
Right now airport extreme model A1143 connected through ethernet to cable modem, airport express connected through ethernet as support. I will remove A1143 and replace with new ac capable airport extreme. Move model A1143 to main house level, keep airport express in basement. Yes all ethernet connections work great.
I will remove A1143 and replace with new ac capable airport extreme.
Things are differnent now if you are planning to use Mountain Lion. Disconnect and power down the old AirPorts for now. The general process to set up the new AirPort Extreme would involve:
1) Powering off your cable modem for at least 15 minutes or longer to allow it to reset. Once this is done, power up the modem and let it run a few minute by itself
2) Connect the Ethernet cable from the modem to the WAN "O" port on the new ac AirPort
3) Power up the new AirPort Extreme for a few minutes
4) Click the AirPort icon at the top of the screen and wait a few seconds for a listing of New AirPort Base Station to appear. Just below that, click directly on AirPort Extreme
5) AirPort Setup will open up automatically, take a minute to analyze the network, and suggest that the new AirPort be configured to create a wireless network. Accept the option and enter the info during the guided setup
6) Test the new AirPort Extreme to make sure that everything is working.
Then, you need to perform a Hard Reset on the old AirPort Extreme as follows
1) Power up the old Extreme and let it run a few minutes
2) Hold in the reset button on the back of the Extreme for 9-10 seconds and release
3) Allow a full minute for the Extreme to restart to a slow, blinking amber light
4) Connect the Ethernet cable to the WAN "O" port on the old Extreme
5) Click the AirPort icon as in the steps above and wait for New AirPort Base Station to appear
6) Click directly on AirPort Extreme to start the set up process
7) AirPort Setup will basically do everything for you....all you need to do is assign a device name that you want to use for the AirPort Extreme
Once the old AirPort Extreme is set up and working, you would repeat the process with the AirPort Express to get it set up
If you have not used AirPort Utility in Lion or Mountain Lion yet, you will be amazed at how easy it will be to set up the old AirPort Extreme and AirPort Express.