How much of a difference would a Quad core be over a Dual core?

Assuming both processors were both the same GHZ wise. How much of a difference would a video game have with a Quad core 2.4ghz rather then a Dual core 2.4ghz. All the videos I have seen on youtube of the performance of an 8800gt generally includes a Quad core, which makes me believe that has a huge part in the performance. I have a AMD Athlon 64 X2 4400+ (2.3 GHz) how much of a difference would something like a Intel Q6600, 2,4 GHz be. This is assuming I'm only using the computer for games rather then applications and random stuff.

I cant afford something like that so upgrading is out of the question. I'm just trying to find something to base my expectations on. Here's the video BTW the guy has many videos http://www.youtube.com/watch?v=ixeVG06gh7M .

First things first: The Athlon 64 X2 chips do not compare clock-for-clock with the Core 2 architecture. Even a dual-core E6600 at 2.4GHz is quite a bit faster than your X2 4400+. Better even than a 3.0GHz Athon X2 chip for that matter. That being said, for current games on the market, the Q6600 is not significantly faster than the E6600. Very few (if any) games properly support multi-threading, although there will likely be a big push for it in the future.

How about memory cache? the q6600 has 8 megas l2 cache that makes huge performance gains in games..geraldus

Not really. Look at the E6600 versus the Q6600. Or better yet, the old E6400 versus the E6600. It's the entire architecture of the Core 2 series that makes it so impressive, not any one feature.