A8 3870k trying to play borderlands 2 with max Physx

Proc. A8 3870K
Ram 4Gb
GPU AMD 6650
PSU Corsair 650
Motherboard ASUS fm1 A75 v pro

So, earlier this year I build a new pc with 3870k APU just to have a budget pc.

Later this year, I saw borderlands 2 on commercial and thought this would be a great low maintenance game.

I was wrong....the game demands very good GPU to get good PhysX effect.

the game is awesome and..I am looking to upgrad my gpu. Its already crossfired with 6650 but Physx quality is really bad

I was looking at hd 6950, but would this bottleneck with my cpu?

Also I haven't overclocked my cpu and gpu yet, (don't know how to oc the cpu ) should I get it overclocked it first?

how would this improve the physX?

Any suggestions and advises are welcomed!