When the average modern computer game first starts up there are various splash screens that appear. Rather than simply being forms of advertising, they tend to indicate what the software itself is capable of making the most of.
The startup screens for 'Crysis: Warhead' for example, feature the usual Nvidia splash screen but also a screen saying 'Runs better with Intel Core 2 Duo inside'. My own E8500 3.16GHz handles Warhead very smoothly indeed.
There are a gradually increasing number of games that make use of the second core, thereby increasing the impact of the game on the user. I don't know of any games at present that have been coded specifically for a quad-core cpu.
It all comes down to product sales - the more copies of a program that can be sold, the better. Until there are more users with quad-core systems than users with dual-core, the gaming industry will home in on the Core 2 Duo machines.
I would suspect that in the global scheme of things, there are far more users with single-core machines than anything else worldwide. Dual-core systems are now being acknowledged by the gaming programmers catering for dual-core implementation within the game code.
I suspect specific quad-core coding is still some way off. :)