If you don't have to play the game on day one, what you could do is simply wait for the game to be released, and ask how the game runs with different video cards. If the game happens to run on high with a 5770 with good results, then get the 5770. If they game is choppy with the 5770, get the GTX 460.
However, I will say, that you certainly wouldn't go wrong with the GTX 460. It is a nice card.