Can someone explain/clear this up for me?

You can't add up the speed of each core. All cores, depending on arch can run at any selected range of speeds. For example, Core 0/1 can run at 3Ghz where Core 2/3 will run at say 500Mhz. This is still considered a 2.66Ghz CPU.

heres an analogy for you, you have 1 car going 100 kph. Now you put another car next to it also doing 100kph. Are both cars now doing 200kph??? no they are both separately doing 100kph each. just like 1 core at 2.6ghz or 2 cores at 2.6 ghz each. However if there are 2 cores doing 2.6 ghz vs 1 core, 2 cores are theoretically capable of doing 2x more calculations than just 1.

+1 for iam2thecrowe. That's a simple, straight forward answer to the question

You mean that if I have a quad core CPU running at 2.66 GHZ, I don't have a 10.66 GHz CPU?  

no more gigawatz for you

Yes, each core will run at 2.66. Multiple cores will be able to do more work at the same time compared to single core CPUs.

Not to make fun out of anyone but you got me ROFLMAO.

I like the Ebay sellers that add em up and false advertise, lol.

I lol at Ebay posters stupidity... I once saw a seller saying a Pentium D @ 3.xGhz was faster than a C2D @ 2Ghz since 3Ghz*2 cores = 6Ghz. lol.