What I mean is, is a 1.66 GHz dual core CPU more powerful than a 2.0 GHz single core CPU?

well, if you use a 64bit OS, and applications and games that can use it, yes.

yeah, providing that the programs in questions are designed for multi-threaded use, then yes

If you mean between the same type of CPU, such as between Core Solo and Core Duo, then in many cases yes, a 1.66GHz Dual Core would outperform a 2GHz Single Core. A modern Operating System will be multi-threaded - Windows certainly is, whether 32 or 64 bit - so the OS itself will benefit from the dual CPU cores for its own functions. Many current applications are also multi-threaded, and will perform better with Dual Core. An occasional program might require multi-thread mode to be activated from a menu option.

Thanks for the info. What about Unreal Tournament 2004? Is that dual core capable?

nope. it'll make use of the HT single core processor tho.

Keep in mind that a single-threaded application can still benefit from being run on a multi core system, as other system functions can be distributed to another core, and parts of Windows that the application may be using, such as DirectX, can themselves be multi-threaded.

@OmegaWolf747

With games there is often before the intro a short commercial like "runs best on dual core/optimized for.." something like that. Together with the usual nvidia/ati commercial. It will give you an idea of what system the game is best suited for.
Dual core is very commen with recent games.
Only crysis run quad core, but I've seen some benchmarks and it's higly questionable. Might get patched?

But besides that, it's like Platypus says.

Just if you had not noticed.