The video cards in the last generation of MacPro's could only be upgraded to the cards offered directly from Apple or the handful Mac-specific parts offered by third parties. With it's unusual form factors, the new MacPro will probably offer even fewer options.
Since OSX 10.7-ish a lot of cards have "just worked". Before then using a PC card was a bit of a crap shoot, but not impossible, usually OK if the card was the same as the reference design.
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I agree with this post except for this part:
Since OSX 10.7-ish a lot of cards have "just worked". Before then using a PC card was a bit of a crap shoot, but not impossible, usually OK if the card was the same as the reference design.
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