Just thought I'd add the solution that i found for this problem 2 Xboxes on one router in case anyone finds this post, amongst the millions all ready out there.
It would just not work with the belkin at all - (even though xbox said it was compatable in the end) -
so in bought another router, which i'd like to say was half the price of my belkin!
Netgear Wireless-g Cable Router - WGR614
bought this on the advice of a friend who has the same isp as us virgin......... and worked right away no setting changes just plugged it in and away we went.