revert back to the older firmware.
Causing bad ip's? DHCP is enabled? if so the router hands out the Ip's (192.168.*.*)......the ISP provides a real world IP/ one only.
try running in wireless G Mode only./ or even B mode only. on the WRT400N.
also have you tryed the Xbox Wired.....like it used to be,
you should have no problems at all with this setup....other than the fact that the more users/ laptop/game players ect are using the internet at the same time the slower it will become.... shared Bandwidth,
I'm not a fan of VX works firmware based linksys routers,,,,,the older ones wrt54g v1>4 are best, with the wrt54gl's being supreme,.