Dual 5770's are generally slightly faster than a single 5870. However, not all games support CF (a lot more than not will support CF). One draw back from going with 5770's is that you have a more challenging upgrade path should you choose to upgrade.
Yes, a single 5870 would get the job done, as would 5770's in CF. Of course that depends on how high of FPS you like, and if you are willing to not have EVERYTHING maxed out with some games. Although, most games will do well with a single 5870 at max, or dual 5770's.
its generally regarded to be a better option to buy a single more powerful card, than two weaker cards for crossfire.
if you get a 5870, you have the option of adding another later on, which is an option you lose if you crossfire two 5770s now.
not to mention the improved heat and power efficiency while running the single card.
for the sake of marginally improved performance in the games that support crossfire, you lose many things.
The crossfired HD5770s will usually give you slightly higher average frame rates but slightly lower minimum frame rates due to the 128-bit memory bus. I'd definitely get the HD5870 instead, but the best value on the high end is getting an HD5850 and OCing the hell out of it.