I would spread out the airports as much as possible. If you can hardwire the remotes back to the main, you will have better performance, the powerline device will work for one, and if you can run an ethernet cable to the other it would be best (or possibly add another powerline, not familiar enough with them to know if you can do that)
If you are mildly handy, it's not that hard to make your own ethernet cables, or to make a wall mount box with ethernet. You should be able to get the supplies and tools at your local home store.
if you can't run hardwire, you can do WDS, but if the repeater is only getting a weak signal, you won't have good results, and even with a good signal WDS will impact performance.
I would make 2 networks "network_N" coming from both extremes and the express, and also have "network_G" from the dual.
as far as which extreme to use for the main (connected directly to your modem), and where to put the express, i would put the dual closer to where you will be doing the most surfing. If you have iPhones or other G devices, they can be setup to join the G only network. so just do whicever makes sense for your house's layout.
and if it makes sense to put the express next to a stereo, added bonus, you can use airtunes.
5ghz signals don't go through walls well so you'll probably want to avoid unless you will mainly be using in the same room (or maybe one over). So i would recommend using 2.4 GHz N. (with B/G compatible if you need it)
on the express (and i'm assuming there's a similar setting on the extreme) you want to put it in "off (bridge mode)" on the internet settings screen on the remotes, then set up the wireless with the same SSID and security, but different channels (or automatic.) Your devices should switch to the stronger signal without any intervention from you.
