What is the 5th Multiplier Bit?

Still trying to decide between A7N8X-DX and NF7S. I read something about Asus only being able to access 4 multiplier bits and the Abit NF7S can access 5. What does that mean in terms of OCing a 2500+? (It's unlocked). I'm leaning toward the Asus, mostly because it seems to be easier to fit the Zalman 7000CuAl HSF. The Abit would need me to mod it.

Obviously lots of folks OC the A7N8X to a great degree but I'm curious how the bits work.