What happens if:
1) The game gets delayed? I got an 8800GTS to play Starcraft 2, but by the time that game came out, I already upgraded to GTX470s.
2) The game is so GPU demanding that it crushes any single-GPU card at max settings? When Crysis 1 came out, it destroyed 8800GTX SLI. Even now 5 years later not a single high-end GPU can fully achieve 60 fps throughout the game. Crytek can easily make a game that mops the floor with the GTX690 if they wanted to based on their track record.
Here is a list of 12 games that use CE3:
http://www.giantbomb.com/cryengine-3/92-2917/games/
I would say, if you don't have a current generation card, grab a GTX670. If you still need more performance, grab a 2nd one next year at a discounted price. If you currently don't need a new GPU upgrade, just keep waiting until the game is released as GPUs only get faster and/or cheaper. I wouldn't buy a $400-500 GPU now to futureproof for some game next year unless you were already eyeing an upgrade in the first place this fall.
Crytek has already stated that Crysis 3 will make PCs cry. Considering Crysis 2 barely moved the bar in graphics over Crysis 1 (and in some ways it looks worse), I have a feeling not a single $500 GPU today will play it maxed out at 1080P.