Idiots question regarding apple networking....

yes you can use an express to extend the range of an extreme, but ...

if your laptop will not get a signal where you want to put the repeater, the repeater won't work.

You'll probably want to avoid using 5GHz mode, it is killed very quickly by walls. I have a couple places in my house, where i get a better signal from a b/g router that's further away than I do from my express with a 5GHz N signal.

if you can get a hardwire to the remote location(s), you can have multiple base stations, and once it's setup you won't notice changing between them as you move around the house.
just put same SSID, and security settings, with different channels (auto is ok) on each base. also you want the remotes in "off (bridge mode)" under internet connection sharing.

One thing that is often overlooked (conveniently not mentioned in 's advertising) is the fact that wireless repeaters kill your network speed, almost in half for each jump, so 2 jumps, and you're running at 1/4 speed on the furthest repeater.