If I'm not mistaken, when you're trying to crack WPA, you need to specify a library file that contains password for brute force attack,

GPU affects cracking process is new to me actually.

Yes, in term of speed CPU win,
but in term who can do lot of work at the same time, GPU is the clear winner,

for example.
the Core i7-3970X is 6 core processor- the price tag, i belive it somewhere near rm3k
and ATI HD 7990 is have 64 Compute Units (4096 Stream Processors) and the price is the same as i7.

when you saw the number of processor core,
6 against 4096, its clearly the GPU have the advantage doing parallel work,
not to mention, you could plug in more GPU in a single motherboard to stack the GPU power.