Ah, Vox beat me to it lol. I would only get the Q6600 if you KNOW you are going to make use of it (i.e. encoding, rendering). For just gaming you should just get a dual core (e.g. E8400) and spend the spare cash on the better GFX.

To explain myself a little better, the 9600GT is just an 8800GT but slightly worse. The 8800GTS 512MB is better than them both (I have this card). Basically, get the 4850 and you should be happy, depending on your resolution. How big is your monitor?