Wifi speed

I have a Dell DW1520 wireless-N WLAN half-mini card in my Optiplex 390 (running Windows Home Premium 64-bit), wirelessly connected to a Wireless-N router (Arris TC8305C).  I connect with excellent signal strength but only at 54Mbs (G-speed).  My Dell DW WLAN Card Utility's site monitor says the router is generating 216Mbs.  Other wireless devices on this same home network connect faster.  Can anyone help me to connect the computer at higher speed?