*....two 3.0 ghz chips which would make your pc have a speed of six ghz*
No - it's always two (say) 3GHz with a speed of 3GHz each. When it's running applications that can take advantage of dual processors (and not all can), then you can't get to double the throughput, but you will get higher throughput than a single core.
I own a PC that has a Intel® Pentium® 4 Processor 950 Dual Core Processor (3.40GHz, 800MHz, 2x2MB cache) I have overclocked it to 4.1 GHZ, I must say that it did not need overclocked, its just that I wanted to get as much as I could out of it, I makes encoding dvd and other processor intense tasks very fast, even in its unoverclocked state. I can't recommend dual core enough, basically you have two cores built into the processor with each one running at that speed, this allows you to say for instance, encode a dvd which would be done with one core and then you have another core which will run at that speed to do other things. Basically you can be doing two seperate things on the computer and it will not slow down like it would if you only had a single core processor.