802.11b/g clients in an 802.11n bridged network
It depends on the rev of Airport Express. The older Airport Express' only extended the 2.4GHz band.

The last Airport Express (before the change that makes it look like a white AppleTV) could extend either band.

The latest rev (looks like white AppleTV) will also extend either band. From what I understand,  if your 2.4 &amp; 5GHz band have the same network name, it will extend the band that has the strongest signal (since there isn't a way to select it in the Airport Setup).

If you have separately named 2.4 and 5GHz network names, then it will extend the network you specify.