How do I optimize my Airport Extreme network ?

I went to the Aplle retailer today to get a Airport Extreme (AE) So I could stream movies to my Xbox, and have acsess my movies and music wirelessly..

My problem is that I the copying and streaming speeds are way to slow to be usefull for anything really, like 2/3 megabytes per second… My macbook and the AE both support the new n categoriced network protocoll, but my Ads modem (wich happens to be relatively new does not…)

Furthermore I tried to connect my Xbox to the AE, and while it connected to the interweb, it did not find any other machine on the network.

So, do any of you flutherites know how to set up this thing the optimal way ?

My gear:
A speedtouch 780wlt multimodem (wifi router and dsl modem in one)

A brand new Airport Express Router thingy

A USB2 HDD connected to the Airport Extreme
An Xbox 360 that needs cable connection (I do not intend to buy the wifi thing since it sits 5” from my AE

Oh and I use rivet to connect the macbook to the Xbox (rivet is a nice litle piece of software that makes the mac visible on the Xbox, for browsing and streaming)

I anyone have a good solution I would appreciate it greatly :D