If using a second AirPort Express to extend the wireless network in your home, as well as setting the new unit to extend a network, ensure that your first unit is allowed to extend: in Airport Utility select the existing unit, click Manual Setup, then the Wireless tab, then check the “Allow this network to be extended” box.
I came across this by accident while trying to set up X-Box Live for my son using an Ethernet cable into the second Airport Express. Although I’d set the second unit to extend my existing network it never gave the improvement I’d hoped – now I know why. Only when trying to work out why the X-Box wouldn’t connect did I discover that the second AP unit wasn’t showing up in Airport Utility, or doing so intermittently.
The above seems to have fixed all my home network problems.
If your network is still slow try a different channel. To this open Airport Utility and click on Manual Setup.
Have a happy, healthy and successful 2011!