Airport Express very weak connection
I'm a big fan of Apple's Wireless Diagnostic tool on recent OS releases (find it with Spotlight as it's not in /Applications).

You can use the Utilities window to view real-time performance and also perform a wireless scan to map all close-by transmitters and see if a suggested channel is better than your current channel for your base station.

The one piece of information missing from your post is how your internet speed test compares to the transmit rate your WiFi adapter is communicating to your base station.

If your signal to noise ratio is below 15 or your transmit rate matches the speed test - then you can work on finding a better antenna / channel or affect thins by improving the radio performance.

If the transmit rate is low - that would reinforce your difference between the WiFi speeds and the wired speeds.

Here are a couple of the better threads to help as well:


What OS X signal strength test apps exist to measure Wi-Fi networking?
A utility for tuning your WiFi network?
Wifi channel selection
OS X status bar wifi icon - meaning of bars?




You can even see how this tool can be used in action in this answer - where I turned on a microwave to show that there was some interference from that device.

If you have an older OS, you can get similar information from the logs and wireless clients section of Airport Utility: (Image from the excellent Anand Tech article on Airport Extreme and Time Capsule - http://www.anandtech.com/show/4577/airport-extreme-5th-gen-and-time-capsule-4th-gen-review-faster-wifi-/8)