How do you compare single and dual cores power

What most users should keep in mind:

Not all tasks can be parallelized. That is a huge problem for multi-core systems, because with unparallelized tasks the second (and third, fourth, etc.) core is idling - resulting in no additional performance from your multi-core system.

Again this is no "it's hard to do" problem, but a fundamental problem. There are a lot of algorithms that are just not working with parallelization - this is mathematically proven.

And even if you can restructure your algorithm so it benefits from more cores it's always an additional burden on the programmer, because he has to control synchronization between the cores (of course only if the algorithms depends on that, but that's often the case).
Restructuring is a process that is currently done by hand, and which is very hard to efficiently automatize.

So don't think that multi-core architectures are the ultimate answer. They also introduce a lot of new problems.

Jam it back in, in the dark.

@RacinReaver: The scheduler of your OS should take care of this. You don't need a multi-core system to have RT multitasking.
That's what process priorities are for. I usually set compiling jobs to lowest priority so they don't affect the overall system performance (and don't produce input lag).

Forcing applications to one core is only a good idea if the apps have problems with multi-core setups (the UT engine for example has some issues, resulting in too slow or too fast rendering).

There's nowhere I can't reach.