Well this is just a heads up really. I had the D-LINK DSL-504G and got a Xbox 360.
This router is crap... even if you open the correct ports for XBOX Live you will still get a NAT-Strict result in the XBOX Live tests. This meant i could connect to very limited number of games and i could never ever connect to my friends game requests that were in New Zealand. After playing round with the damn thing for a few days and googling alot i found alot of people had this issue and that the router was just really budget and its NAT settings could not be changed. I now have the D-Link DSL-G604T (i felt like a right knob buying this since the other d-link gave me issues) but yea it worked. I now get either NAT-Open or Nat-Moderate (it seems to change its mind everynow and then!) but anyway i can now connect to all friend requests and a lot more games are avaliable to me. Im not saying its the best router/modem for the job but it does work. Just thought i'd post this up incase others are having issues with their DSL-504G on Xbox Live. (alot of people have these as they were the xtra standard a while back)

Cheers