Try setting all the devices to use B, and see if that helps. Also, try
to use a different channel, and enable MAC filtering to ensure that no
one else is connecting to your wireless.

Are you using WEP or WPA? There is some overhead associated with
encryption, but it shouldnt be causing that much difference in speed.

If you disable all the other devices, and only connect one wireless
device does your speed improve?

Try to remove as many external factors as possible to troubleshoot this
connection.

-Luke