Think of the AirPort Extreme as a fancy AirPort Express that will allow you to connect a USB hard drive to the port on the back of the device.
It sounds like you might have a Time Capsule already, so you could use the AirPort Extreme to connect to the network using either Wi-Fi or Ethernet. The wired connection is always preferable, but it may not be easy.....or even possible......to run the Ethernet cabling from the Time Capsule to the AirPort Extreme.
In that case, you will need to locate the AirPort Extreme where it can receive a strong Wi-Fi signal from the Time Capsule. If your current AirPort Express is connecting using Wi-Fi and working well, then an AirPort Extreme in the same location should work well.
Keep in mind that you will need to add a USB drive to the USB port on the back of the AirPort Extreme if you want to be able store data or back up to the USB drive connected to the AirPort Extreme.
So, if your Time Capsule is already backing up using Time Machine, you could set up the AirPort Extreme as a secondary backup location and have dual backups.