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I would have thought utilisation of both 9400 and 9600M GT would occur, only in Vista though. I know that the new Dell Studio XP 1340 has hybrid SLI with an integrated 9400M and 9600M GT enabling switching dependent on use. Boot camp unlike parallels, your running a proper OS install with nvidia drivers unless it's todo with the actual chipset & motherboard. Otherwise using the 9600M GT over the 9400 would make sense.
 
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