Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
The top n speed in "mixed mode" is 130mbps. In n only mode its 270mbps. I run a bridged b/g and n network where my old router handles all the b/g stuff and my AEBS handles n only. Exactly as paragonj said. This is the only setup that will give you maximum speed unless you hook your XBox up via ethernet and dump the old router.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.