The HD5770 is #27 on the graphics performance list
Its up to how much you want to spend, I'd say if you want more performance than crossfiring 5770s you'd want to buy a card that is #12 or higher. If you don't want to spend that much on a new GPU, then buy a 2nd 5770.
(5770 x2 isn't on the list, but the 4870 #26 is at #13 in crossfire)
Yes, if its an OEM version of Win7 it is tied to the motherboard. I have heard stories of people managing to get Microsoft to allow them to transfer the existing license, but there may have been extenuating circumstances involved.