Fifty percent of your wired Internet speed using Wifi on the 2.4Mhz band is about par for the course.
Run inSSIDer and see which of the router radio channels 1, 6 or 11 are least busy in your neighborhood and try each in turn to see if there is any improvement and by doing that and perhaps a few other tweaks getting to 30 will be really good.
If your MAC has a 5 Ghz capable WiFi adapter consider hanging a 5 Ghz AP off your Actiontec router and perhaps if you are in the same room you will get to 40 Mbps.
WiFi is for convience and will never be the fastest connection.