and intel core 2 duo that means the ghz processor is really 4.0 right ?
dual core is a bit similar to dual CPU. it allows more calculations to be performed at once, but does not increase the speed at which the calculations are performed.
if you have 4 people working with calculators on some problems and you go to 8 people working with calculators on the problems, the speed at which each problem is solved will not increase. the time it takes to solve problems are solved will be faster.
for a long time, most operating systems weren't multi-thread capable but i believe that is changing now, so OS's might be able to make games run better with a dual CPU, but i wouldn't expect much of a performance upgrade in gaming by going dual core vs. single core.
if you were rendering a 2 hour video file, the dual core/cpu would be about 1.5-1.7x faster than the single core.