In addition to what Tesserax has mentioned, the performance of your network will actually suffer significantly if you install the AirPort Express......since extending a network using wireless will always result in a loss of up to about 50% of the potential speed on your network. Here, I am assuming that the Express is set up to wirelessly extend the signal from the AirPort Extreme.
So, you will likely do better by not using the AirPort Express at all.....unless you need it for some other function on your network.
If you still want to use the AirPort Express.....and you want to prevent the speed loss from occurring on your network......then you will need to connect the AirPort Express to the AirPort Extreme using a wired Ethernet cable connection. Even then, the speed potential of the AirPort Express is quite limited compared to an AirPort Extreme, so if you want to maximize the performance of your network, you will need to replace the AirPort Express with an AirPort Extreme.