From what I know SLI needs identical GPU's, but with Crossfire you don't ( I believe).
So you would need 2, 480s or 2, 580s etc for this to work.
So go for the 2 480,s but if your budget allows it I would get 2 580,s.
Or just go for a single 580, your call, another factor running the 2 cards would be heat so make sure you have plenty airflow.
Ben
PS. wait for other comments from those who may be more knowledgeable regards these GPU's.