I recently bought a Linksys 610N and love it, although I hear the 310N is also a great product.
Gigabit internet will help your connection bandwidth between devices in your house, but not really for your internet connection. Even your wireless bandwidth is greater than the dsl 6 Mbps.
I have an express that connects wirelessly to the 610N as a client. It works great. You may also be able to set up WDS in the router’s configuration, which allows the routers to extend the wireless range. I remember a while back with the 54gs, there was a whole rigamorerole you had to go through to get it working, but these problems may have been ironed out in further Linksys software. You should google “airport express wds [router model]” though before you buy to make sure.
I can’t comment on the Airport Extreme backup options.