Wow... Microsoft fail.
Check out this article:
http://nukoda.com/news/hardware-revi...ess-n-adapter/
They described the problem and how they fixed it. Maybe it might work for you?
Any and all software updates should come through a system update - why is Microsoft packaging an installation CD with a console device? Makes no sense to me...
I just plug my Xbox into an Apple Airport Express which functions on the N protocol. I'm relatively sure that gives me "N" speed but who knows... maybe Microsoft limited the Ethernet connection?