I also would go with a single 5850 even though two 5770s are faster and sometimes cheaper but the main thing is that you have an upgrade path with the 5850 since it only takes one pcie slot.
If the game does not support Xfire properly you are stuck with one HD5770 (sucks).
If one of them goes bad you are stuck with one HD5770 (sucks also).
Noise and heat.
I vote for Crossfire the 5770s. More bang for the buck with a little OCing mine beat 5870s, plus if you don't plan on upgrading for a while when you trade them you can probably trade both for a single 6XXX card then crossfire them...