The only time I would buy a pre-overclocked video card is if the price was not much higher (and I mean under $20 difference). To start off, theres no way to tell a video card has been overclocked since it's all done from within Windows-based software. Secondly you can usually achieve higher overclocks by overclocking a factory clocked card yourself.I disagree with the person above me. Lots of warrenties get voided for OCing, but if you buy one already OC'd then A) you don't have to and B) the warrenty covers OCing.
The only real difference is the warrents, places like evga have lifetime warranties, and others have 3 year ones like XFX. Although most people will upgrade within 3 years, some people do keep it for longer. EVGA also has a nice step up program which is a nice thing to have.