Plus, a lot of games aren't limited solely by the processor. A high requirement for the processor usually corresponds with a need for a higher end graphics card. If you don't have that, then there's no chance that your computer will be able to play it properly.
And either way, you can't just multiply 1.67 by 2 to come out with your new "dual core" speed, it just doesn't work like that. If the game is old enough, and it's saying it needs 2.0 Ghz on a Pentium 4 cpu, then your new Core 2 Duo will be fine handling it. Trying not to get into details, Core 2 Duo just outperforms the Pentium 4 if they have the same Ghz rating.