Whoa, DMZ? Why? You should NEVER DMZ anything. It's not a good solution. Besides DMZ bypasses QoS and I believe thats what he's trying to do.
He was half right, however. It's best to have the XboX have the same IP all the time. But you dont need to static it, the router has DHCP reservation. This allows you to enter the MAC address of your XboX in to a field and give it a desired IP.
Then router will then ensure that the XboX always has this IP.
Next is QoS, here is a FAQ on configuring that.
If you still need help post again.
http://www.support.dlink.com/faq/view.asp?prod_id=2485&question=DIR-625%20/%20DIR-635%20/%20DIR-655%20/%20DIR-625_revC%20/%20DIR-628