Yep I'm thinking if I ever need an upgrade, I'll go Quad core which is why I bought one of the cheapest CPU's when I bought my Intel parts upgrade.
I would say that yeah you can get more juice out of the Q6600 for example for gaming than the E6300 simply because the Quad has 8MB L2 cache total E6300 has 2MB. (Yeah I know theres currently no games that use all four cores and just some use two...right?)
I would say that your GPU is allright for a long time, it's on par with the 8800GTX so you shouldn't be thinking on a video card upgrade for this year IMO. If you wanna upgrade GPU, wait until you just can't play anymore with the 8800GTS, when games start making you lower those settings, then get a nice up-to-date GPU, 9000 series I guess.
You dont need a new gpu or a new cpu yet, we shall seee when the time comes closer, but atm, you wont get any realy performance increase in anything getting a quad, unless you find an app that is multi core intensive