What is best GPU for WPA hack?

[omidh]

Hi, I want to buy a GPU (or maybe more) only for hack, but don't know what to buy. I want only use it for hack so i don't want waist my money on graphic quality (games).
And what i should look when i want to buy a GPU for hack? Engine Clock, memory clock, memory bandwidth or ...?
Does a GPU with 1GB memory compute faster than a GPU with 512MB memory?
My price range is about 700$

Thanks