My bro (lives in NJ) and I (live in AZ) play Top Spin probably once a week, no probs. Sometimes, Crimson Skies too. Every once in a looong while I play Halo all on XBC. So I know it is working.
My question is, why doesn't Links 2004 or Serious Sam 1 or 2 work? We can never find each others box when we try. The games are listed as system link capable and when we go to system link it never finds the others host.
Any help, I am willing to try anything. Thanks.

Try refinding your xbox on your network with that game in system link mode, some games - I know halo 2 can do it, mess around with the MAC address on your xbox, I would say try searching for it again and see if it comes up with a different number, also a solution to try would be to unfind your xbox - delete it from the list, and then refind it.