I bought this card for the mac pro 2008 (swedish site, but you should be able to see the specs at least):
http://www.prisjakt.nu/produkt.php?e=1240276
That Zotac GTX 680 with two 6-pin power cords works awesome with a 1440p screen and it will surely beat the crap out of a 5770 at 4K, hands down. 4GB memory is nice for that bigger screen.
You don't get a boot-screen though but otherwise it works awesome out of the box.
// DL