A simple form of calculation for what the speed advantage "could" mean for some of us:
4-Core at 3Ghz: 4x3000= 12000 Ghz
8-Core at 3Ghz: 8x3000= 24000 Ghz
But THIS is highly software dependend!
Most of us would not even notice the difference. Only if you use Final cut Pro, and more specific "Compressor" to render video to Mpeg 2, or if you do high end 3D animation (I use E-on's Vue Infinite a lot for example) Then you will see the speedbump.
That's also the reason why apple did not put in any effort to make custom 2.0Ghz 8 core's, or 2.66 Ghz 8 core's. People who REALLY need 8 core machines, also can use that .34 Ghz extra, and are also really willing to pay for it.
If MS word would open up in 2 seconds, or in 1.6 seconds, you will not notice the difference. BUT, when you do a Vue Infinite Render of 8 hours straight, like I'm doing nowadays with my Dual G5 2 Ghz, I will see a theoretical speedbump of:
2xG5 2GHZ = 4 Ghz
2x 8core 3Ghz= 24Ghz
6 times!!!
So my render will be finished in 1 hour and 20 minutes, instead of 8 hours.
Whiiiihaaaaa...