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Connecting with this G device WILL slow your network to 54mbps.Does it connect in without encryption? Where you using an ASCII or Hex WEP key? How far from your router is your DWL-G810? The fact that your network is slowing down tells me you are associated, you are just not passing encryption correctly.
No that should be the best course of action for you as far as I can tell. Remember to turn off DHCP on the second router and connect them LAN to LAN to prevent double NAT'ing that XBOX.