I remember the official exchange places having around the same rates, whether they were in the city or airport. At several hundred dollars, the miniscule differences won't make a difference.

I agree about using the ATM's to get your cash.

A word of warning: The police actively warns people to not exchange on the street. There's a rash of cons involving fake currency. I remember there was even a large police booth set up in the main square about this. Interestingly enough, I was only approached once.