Older Airport Extreme with new Airport Express

Use the AirPort Extreme as your "main" router and the AirPort Express as the extender.

Since you will be extending using wireless, the AirPort Express will need to be located about half way between the AirPort Extreme and the general area that needs more wireless signal coverage.

Another way of saying the same thing is that the Express can only extend the quality of signal that it receives, so it must be located where it will get a strong signal from the AirPort Extreme. Try for line-of-sight between the AirPort Extreme and AirPort Express as much as possible.

AirPort Setup in Lion, Mountain Lion and Mavericks will literally configure the Express for you automatically, so other than entering a device name, the work is done.

Surely, you are not using OS X (10.4.11) as you indicate in your profile, correct?

Your Extreme should still provide better overall range over the 2nd gen 802.11n AirPort Express Base Station, so I would still recommend that you leave the Extreme as your "main" router.

You will want to use the following instructions to extend the wireless network from this Apple Support article.

The key to success if extending the Extreme is to place the Express within a 25+ dB SNR range of the Extreme as it can only repeat the signal with the same bandwidth that it receives it. If the signal is very weak the Express may not even be able to extend it at all.

Check out the following AirPort User tip for details on optimal base station placement. Please post back your results.

Please update your profile so things will be correct when you ask your next question.

Thanks.