The AirPort Extreme will provide more signal to the last floor if the AirPort Express can't provide enough signal, and it's completely logic.
You have to set up the AirPort Extreme using the same way you set up all the AirPort Express Base Stations you have > http://support.apple.com/kb/HT4145
Thanks for the fast answer, I just wanted a confirm before to buy it.
Looking now the data in the airport express I see:
data rate: 86Mb/s
RSSI: -62 dBm
Probably if I but another extreme I will have more, and people connecting to them will have more than the currently data rate to 14Mbits.
Last question: I usually use airport utility and I select "network mode - extend a wireless network".
Is that the correct procedure that I will have to to with the new extreme or is a better way (since the extreme is a router and not only a repetitor?.