>> Keep in mind that switch could be fatal for the graphic card...
>> 
>Could you elaborate on this? The 840 is onboard video, the 7300 is
>onboard video but also with the Apple PC Compatibility card.

I'm going to say mistakes... I never have seen any such case but
the cards and the screen take a shock when you switch so, it's
written every where that it is dangerous. Probably for legal reasons
in order to don't have to pay back some users in this case...

Keep in mind that your material is already shocked (by surges)
often even if it doesn't burn out each time ;-)

pc