Hi there
I used a site called cpubenchmarks.net which boils alot of different tests down to one performance score.
A6-3620: 2,890
Athlon 651K: 3,652
That would give you a ~26% increase in performance.
To compare, a newly built budget pc's cpu, like the Athlon 760K scores 4,570. It's considered to be "just enough" when playing something like Battlefield 4.
It would defently boost your fps, though it's going to depend upon the price whether it's worth it. Else you might want to consider a motherboard + cpu upgrade to see a big difference.
Hope this have been helpful!