I'm a bit suprised there haven't been more Mac Pro user responses here to directly answer your original question... but yes, I use an 8800 GT on my Mac Pro (early 2008 8-core). I know you asked about the first gen (5000X chipset), but as you know by now there's an upgrade kit to bring you up to the 8800 GT.
Anyways, to try to answer your question, I use the 8800 GT to drive dual 24" monitors running OS X and it works just fine. Calibrates fine, but that's tied in with monitor performance, so I'm not sure exactly how much the card is responsible for. Photo apps run faster than my previous OC'd PC with an 8800 Ultra.
I also run Win XP via Boot Camp to play PC games. In this regard, there's a noticible performance decrease from my previous PC/ 8800 Ultra/ Vista combination.
Is that what you're looking for? Hope it helps.