So if I read this thread correctly I can connect an AE with cable on the second floor (UTP is built in-wall) and setup the AE to bridge mode and it would work automatically?
Not sure what you mean by "automatically" here.
You will need to manually configure the AirPort Express to.....
1) Create a wireless network using the same name as the Gateway wireless network
2) Adjust the wireless security setting on the Express to exactly match the wireless security type that the Gateway is using
3) Use the same wireless network password as the Gateway network
4) Setup the Express for Bridge Mode operation
It is 2) that throws most users off, since few know the exact type of wireless security that their provider furnished modem/router is using. They guess at the setting for the Express, it does not match, and you wind up with two separate networks each requiring their own log on to connect.