Core 2 duo has two cores and core 2 quad has 4 cores, so it's like having two or four processors in one respectively. Also the 4mb and 8mb Caches is how much memory it could hold at any given point in time, now you might think 4/8 mb isn't a lot, but for what the processor does it's quite a bit. i would personally take the quad just because it's newer and therefore won't need upgrading as soon as the duo would. and also it's more powerful
the rest of that computer should allow you to play all the new games as well, maybe not on the highest settings but close to them.