I should have clarified. my apologies.
it depends on your situation. it is my impression that a factory oc'd gpu is a card with a better cooling apparatus, ie. beefier. if you are satisfied with a quality and mostly quiet air cooler, then a tad more money is worth it. if intent is to use a better than reference cooler, get whichever 580 reference you prefer. again, that is my impression of what a factory oc gets you. frankly, I have my doubts, but such is the word on several forums. otherwise, a factory oc would be a ripoff that would sour the gaming crowd for guilty vendor. for example, the black ops oc is lame from the reference 772. yet I can easily, no voltage change and comfortably noise-wise, run at 880. by the way, at default this card, and a reference slightly oc'd, will get you ~640-650K.
so, if you have slick aftermarket cooling, get your preferred reference and enjoy a darn good card. if not interested in messing with the card's casing, I can attest the black ops 580 is kickass.
by the way, I am not one of those fanboys. I have two oc'd 5870s in my other machine. I mention this so that you not thinking I am pushing nvidia over amd.