Well personally, I don't see much use for a quad core.. No games are really taking advantage of them, much less dual core, so the only use you will have is if you want to run multiple SETI / Folding @ home type programs. If it's a matter of not costing a ton more than a dual core, then I guess it doesn't really matter, you can buy it if you want; just don't fall under the illusion you're going to get full use out of it. You could manually shift programs onto different cores in task manager, which may be useful if you wanna play a game and free up some CPU power, but that's about it.
If you can, try to get the 9600 GT over the 9600