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You don't use screws in the MacPro, there's a little bracket that clamps down on all the pci cards.
 
okayyyy.... would you be willing to link me to it or at least let me know if that second link I posted was it then?
 
The second link you posted does look exactly like the one in my 2008 Mac Pro, but then again the ad says it is for the 2008 model. I don't know if the same bracket will fit a 2006.

The bracket is probably more for convenience than a requirement. I would think standard 6-32 screws for each bracket location would work too, as it does in almost any PC tower.
 
That is the right type of thing, but that bracket in the second link is for a 2008 MP. I don't know if it is the exact part you need for your 2006
 
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