To be honest with you, if they are trying to charge you substantially more for a pre over locked card, it is not worth the extra cash. You can easily overclock a stock card to meet the same standards and even exceed them on your own. The reason they sell multiple models pre over locked is simply for the money. Essentially they are overclocking something for you to save the hassle but the price difference dosent usually make it worthwhile for the consumer.
Someone may not be able to afford a GTX580 but has enough change for an overclocked GTX570.
Stock cards are great. They are the cheapest option for most people who may not be as concerned with squeezing 1 more fps out of a card. Others like to buy stock and overclock it themselves.
Overclocked cards generally have a higher Core and Shader clock speeds. However you have the potential to have a louder and hotter card than a stock solution. Vendors try to counter this by adding there own cooling solutions. Some vendors do a good job others not so much.
A super clocked 570 may be comparable to a 580 in some benchmarks but at the end of the day it has less stream processors or less through put than a 580.