Yes, it stacks with your str bonus
Power attack says:
Benefit: You can choose to take a –1 penalty on all melee attack rolls and combat maneuver checks to gain a +2 bonus on all melee damage rolls. This bonus to damage is increased by half (+50%) if you are making an attack with a two-handed weapon, a one handed weapon using two hands, or a primary natural weapon that adds 1-1/2 times your Strength modifier on damage rolls. This bonus to damage is halved (–50%) if you are making an attack with an off-hand weapon or secondary natural weapon.
When your base attack bonus reaches +4, and every 4 points thereafter, the penalty increases by –1 and the bonus to damage increases by +2.
This means that at BAB +0 you take -1 to hit and get +2 to damage (+3 for 2h), at BAB +4 you take -2 to hit and get +4 to damage (+6 for 2h), at BAB +8 you take -3 to hit and get +6 to damage (+9 for 2h), and so on.
If you happen to have 20 str (+5 modifier) attacking with power attack, you get your normal modifier with 1h weapons and 1.5 times your modifier with 2h weapons (+7). Example, at BAB +4 with 20 str, you get a total bonus to attack of +7 (BAB +4, +5 str, -2 power attack), and a total bonus to damage of +9 with 1h weapons (+5 str, +4 power attack) or +13 with 2h weapons (+7 str, +6 power attack).