As far as I know impact is a multiplier for the base weapon damage. Then total damage is adjusted by enemy armor and resists, before finally being adjusted again based on your level compared to the enemy.
That's just for PvE. I imagine crucible/IB has a similar formula.
In Crucible all that matters is Impact. Attack has no effect on damage dealt. A brand new Red Hand IX is going to do the same amount of damage in PvP as a fully upgraded one, for example. IB is the same but with level modifiers applied to armor rating, a lvl 20 is going take the same amount of damage as a 32 but his health pool will be lower, thus making him easier to kill. At least I'm pretty sure that's how it works.