Folks who review graphics cards tend to say factory OC's perform a bit better than stock, but the difference isn't often noticable and the price may lower the value (frames/$) of the card itself.
You *will* generate more heat, that's a necessary consequence of the OC.
However, the card itself may or may not reach a higher temperature than a stock card, depending on the performance of its cooler. The heat will either stay in the case or be exhausted, depending. The vidcard fan will likely run faster, but not always, and that often will generate more noise, which you may or may not notice. When a factory OC runs faster at the same temps, part of what you are buying is the changes to accomplish that.
What I buy depends on the price difference between stock and OC'd cards at the time I purchase. If the difference is more than a couple of bucks, I also want to google for a quality review of the card (not users, or reviews that cover only the carton and connectors). If the price of the OC is less, and its a good manufacturer, I'd probably buy it knowing I can likely reverse the OC if necessary. I do NOT depend on OC to get the performance I need, I'd rather move to the next higher card.
Without a quality, comparative review, you cannot tell whether one stock card or OC'd card runs hotter or louder or noticably faster than another stock or OC'd card.
Hope that helps.