It depends on the game that you play,however currently most of the games benefit from a dual core CPU,but some games like GTA IV and some RTS games benefit from Multi-core CPUs so as some tasks like PhotoShop,3D Rendering,multitasking and etc so as i said it depends on what do you want to do.
However if i had to choose between them i would choose a Q8400 and OC it because you can OC a Quad core(Mostly) but you can't add cores to a Dual core CPU.