Dolphin already support quad cores,to be more precise,it uses 3 cores.
There are lot of games which does not have proper sound when using default HLE sould plugin. In that case you have to use LLE plugin. Problem which then arrise is that LLE plugin is very cpu damanding and many games become unplayable. Solution is to use option "lle on thread" which actually uses third core on the cpu. If you dont actually have third core in your cpu,you screwed !
So I would def suggest you choose i7 option...
There are lot of games which does not have proper sound when using default HLE sould plugin. In that case you have to use LLE plugin. Problem which then arrise is that LLE plugin is very cpu damanding and many games become unplayable. Solution is to use option "lle on thread" which actually uses third core on the cpu. If you dont actually have third core in your cpu,you screwed !
So I would def suggest you choose i7 option...