From your user's manual page 29 ftp://ftp.dlink.com/Gateway/dir655/Manual/dir655_manual_100.zip
Under your Wireless settings:
"802.11 Mode: Select one of the following:
802.11g Only - Select if all of your wireless clients are 802.11g.
Mixed 802.11g and 802.11b - Select if you are using both 802.11b and 802.11g wireless clients.
802.11b Only - Select if all of your wireless clients are 802.11b.
802.11n Only - Select only if all of your wireless clients are 802.11n.
Mixed 802.11n, 802.11b, and 802.11g - Select if you are using a mix of 802.11n, 11g, and 11b wireless clients."
Make sure the highlighted one is selected and your n devices will work at n-speeds and you g devises will work at g-speeds.
EDIT: By the way, you have a new firmware that came out last week that fixes speed issues:
http://www.dlink.com/products/support.asp?pid=530&sec=0#firmware
