i am assuming u already have a wireless adaptor for the xbox
a wireless N adaptor would only be of benefit if your router is wireless N compliant
i think the first things to look at are can you move the xbox or the router to create a better connection or can you hardwire IE run a cable to the xbox. A Wireless connection has so many variables and things that can and do interfere with it.
1st step would be as above if you can move xbox or router do so and experiment as to where the better connection comes from . If you can hardwire it doesnt come better than that
hope this is of some help