Enchanted swords can receive two levels of Knockback while bows can receive two levels of Punch. How far do these enchantments knock the enemy back and how does it compare to the knockback from normally hitting and/or sprinting?
After a significant amount of testing, of which GLaDOS would be proud, we managed to come up with a fairly simple table of data from which to formulate an answer.
Variation of about .5 blocks
To reach these results, I first enchanted my weapons until I had Knockback I and Knockback II (note: the minimum modified levels for these enchants are 5 and 25, respectively).
I then had my associate spawn Zombies in the Knockback Testing Super System™ so that I could attack the Zombies from a safe, secure location.
We first tested a Normal Sword (no enchantment) from a standing position, and then again while sprinting. We then repeated the process with enchanted swords. Once we reached a definitive conclusion, we moved on to testing with Bows and their related enchantments (in this case Punch). The results of our SCIENCE can be found at the top of this answer.
I hope this helped!