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I think that technically the nvidia card is a little bit faster. However, I would still go with the ati x800 simply because it takes up one slot in the powermac g5 not two like the nvidia one does. Also you might want to consider the ATI x850, it should be out by now.
 
X850 is barley better. Not worth 100 extra bucks. I just want to play Doom 3 my system can NOT handle it.
 
I prefer ATI to NVidia. Although they might not be the fastest right now, they still make competive products.
 
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