what size is the stock case fan?
has anyone added an intake fan? i think you could add a 70mm intake fan on the left side of the xbox (looking at it head on from the front). the motherboard doesn't have any really obtrusive things sticking up there, and there's enough vents to suck in air. or you could cut the metal.
having an intake fan, and upgrading the exhaust fan would help a lot with cooling, wouldn't it?