If you are unsure about overclocking yourself, you should probably go with a factory overclocked card. The manufacturer will back these stated boost clock speeds. However, the performance between a non-overclocked and a slightly factory overclocked card is pretty negligible. In fact, even when comparing the best overclocked cards with their base counterpart, you will only usually see about a 10-15% difference.
What does this amount to in real world FPS?
Stock: 60FPS -> 10%: 66FPS
Stock: 60FPS -> 15%: 69FPS
Stock 120FPS -> 10%: 132FPS
Stock 120FPS -> 15%: 138FPS
and this is only comparing the stock clocks to the best factory overclock (usually a $100-$150 difference).
With only a slightly factory boosted card, you will see even less gain.
Of course, you can always overclock the card yourself, there are many guides YouTube or blogs that can be easily googled. Overclocking the card yourself will usually get you pretty similar performance gains as long as the coolers are the same as the factory overclock card you are looking at.