I apologize in advance if this question can be answered elsewhere in this forum, but is a multicore CPU's speed based on a single core or is it the combination of all cores that determines the final speed of the CPU?
Each core runs at the stated speed, however a quad core at 3GHz will not perform the same as a single core at 12GHz would. Most applications are single or dual threaded so they can only run on one or two cores, but having multiple cores will allow you to run multiple apps simultaneously so while each task will take just as long as it would on a single core at the same speed you can get more tasks done in the same amount of time.
Both cores run at the same speed. Newer cpus use amd's "cool and quiet" or intel's "speedstep eist" software to automatically lower the speed at idle or low power applications. You can disable both in most bios'. My amd dual core is rated for 2300 hertz, but runs mostly at 1000 to save energy. The speed increases automatically as needed.
Anyways, a system's FSB speed (200 is what mine is by default) is multiplied by the CPU's multiplier (mine is 14 by default) which would give me 2.8Ghz. So if I up the FSB to 250 and leave the multiplier at 14 would give me 3.5Ghz, etc.