Wow I have the EXACT same setup and I also can't get the Xbox to connect in 'N' mode. At best, I can only get the Xbox to connect at 'G' but that's only if I have the DIR-655 router set to G&N mode.
I'm currently using Firmware Version : 2.00NA , Wed, 12, May, 2010
Somebody please PLEASE help!
I really don't want to have to buy something else as a solution to this problem.
Thanks for the space.
Tony