Could someone tell me what the damage would be for a Quarterstaff used as a two handed weapon instead of a double weapon?

Also, what are the Dmg (S) and Dmg (M) listings in the weapon guide?

I was hoping to go primarily Quarterstaff with my Bard, but it's listed as a double weapon. If I can't wield it as a double due to the penalties, and wielding it 2h reduces it to 1d4 damage, I might have to give it up. Hopefully it'll be 1d6 at least.