It depends on if the game was written to utilize more than one core in the first place.
Budoka: I'm an AMD fanboy, so all of my instincts are pointing towards telling you "DON'T BUY INTEL!!!!!11111oneeleven" but the Core 2 Duos are really the best processors currently on the market, so if you don't have a problem with Intel there's no reason why you shouldn't go for one. And I figure if you're gonna go for something like this, you might as well go all out, so grab yourself the 6700.
Gecko: The rated ghz number is per-core, so to get an accurate number on the capabilities of the processor, multiply that number by 2. Overall ghz don't really matter anymore anyway as a way to make a comparison of processors, for reasons I'm about to tell Pyrion.
Pyrion: Actually, even if only a single core at a time is utilized by itself, dual cores are better performers than the old single core procs, because of the better architecture used by the dual cores.