Never buy a factory overclocked card, unless it comes with a nice bundle or something.
The most cost effective thing to do is to buy the cheapest, stock clocked version and overclock it yourself.
The only thing you need to watch for are the warranties. Not all companies like it when you overclock. EVGA is one company that WILL cover your card, no matter what you do in terms of overclocking. I'm fairly certain Sapphire is the same way.