From a % standpoint, a Factory OC card will likely OC less because all 8800GT cards will have the same theoretical max.
Each 8800GT, however, will not OC the same.
Some slightly. Some quite well.
If you want the highest clock possible, you might want to just get the one that is already OC'd the most if you can afford to do so since you will know you have a good chip. Otherwise you are taking your chances.
However, I could not pay too much extra for a lightly OC'd card.
I would just do that myself.