My two-cents , to those holding out for a 5870. I've now officially had all 3 cards from Mac, 4780-5770-5870 , I do 3d/graphic design and I'm also an avid gamer. I currently own a 12 core system and here are my thoughts.
4870 handles 3D better than 5770, it's a solid card although a bit sluggish.
5770 is just awful, better than 4870 on paper but this not a card worthy of a Mac Pro.
5870 it's fast, for gaming. Not worth the extra money for 3D/gaming currently. It's better all around but for those holding out, hold out for something better. The gains on paper were much greater than the gains while using it.
Short: get a stock 5770 or stay with 4870. Hold out for better tech, it's coming soon. Avoid the apple tax on this one.
EDIT: Is their any reason why we don't have a 2gig apple card available to apple users in 2010?