Well, how do your games play? Notice any struggling to maintain playable framerates anywhere? Have to turn anything down to get something playable? If the answer to any of these questions is "Yes", then it may "behoove" you to go crossfire.
Giddyap!!
Then you might as well wait for the prices to drop to pick up a second 5770.
If none of your games drop below 60 fps then don't do it.