Dual HD5770 are faster than single HD6870.
But single card gives you less problem, less heat and noise, and more future proof because you can add another one later in the future.
I would say wait for a while until 69xx series out, after that you can sell your current HD5770 to get new high end card...
I still dont get it how can a single 6870 be more more future proof than 2, since, from what I was told, you can only CF with boards from the same prefix (ie 68xx). Series 69xx is coming right now so it seems it wouldnt matter picking only one 6870 if he wont be able to crossfire it with the news 6950/6970 anyway!