So now were labled "Mac people" huh? I guess we're not human like you are. Do the math, 4000 Mhz minus 2000 Mhz equals???! 400Mhz which is the same as .4Ghz. Dual core or single it's still 400 Mhz.
Actuallly, I'm a Mac person as well (24" iMac, 2.0 White MB, Mini, had a Mac Pro at my old job).
It just seems as if there's a few silly ones here.
I'll walk you through this in small steps.
The processor has two cores.
For all intents and purposes, the cores are basically individual processors (bla bla except for memory architecture bla bla).
Simplified, this means that a 2GHz dual core processor will give you...
2x2000MHz.
This means that a 2.4GHz dual core processor will give you...
2x2400MHz.
Now, I may not have gone to the same fancy school as you, but my maths teacher taught me that 4800-4000 is 800.
If the entire platform were single threaded, the extra core wouldn't matter - but since it's not, it means you get 2x400MHz extra.
Now go back to being silly.