The Airport Extreme uses wireless N technology, meaning it is both fast and has a wide coverage. Provided that you only let N enabled devices connect to it.
If you connect a G enabled device, such as:
- an iPod touch/iPhone
- Nintendo Wii
- Older Mac or PC
the Airport Extreme along with any other router will "slow-down" it's connection to allow the G devices to connect to it.
I currently have my network set-up with a Time Capsule and another router, which acts as a modem for the TC, but also broadcasts its own wireless network. Meaning I have two networks in my house, 1 N and 1 G. You can do this with AirPort's "Bridge Mode"
See
this post on how to set-up a Time Capusle or AEBS with another router to have 2 networks in your house.
Therefore, if you let G devices connect to your AEBS, it will slow-down and will derease it's network coverage.
If you have a Mac that has 802.11n built-in but it's not enabled, you can also purchase the wireless N enabler from the Apple Store
here