Haven't read all the posts in this thread, but I just upgraded my Hac today with a EVGA GTX 550 Ti and it worked out of the box. I had Intel integrated 3000 video before, which gave me artifacts. Those are gone. The only issue I had was a panic upon reboot. I decided to rerun MB v4.3 and specified the 10.7.2 Nvidia option (along with my other selections) ... I rebooted without incident. I guess time will tell if this is truly safe, but if everything is stable, then it was as simple as 1) install card, 2) tweak bios to look at new slot if not on auto, 3) boot up, 4) run MB, and 5) reboot.
I picked this card because it was supposed to be OOB compatible. It wasn't coincidence that I have the card I have. After messing with wifi, I place a lot of stock in picking extremely compatible components for my Hac!