BleepingComputer.com: i7 turbo mode vs core 2 duo

How should one compare a core 2 duo and an i7 with turbo mode?

I, and I guess most people, dont need the 4 cores and Hyper-Threading which i7 offers. I think a fast and good 2 core will be just fine. However, I would like to have the opportunity (as an insurance for future needs) to use 4 cores. Will i7's turbo mode (which automaticly turns off one or several cores and increases the clock rate on the working cores, given that all 4 cores arent needed and the power usage is below maximum) make the i7 into a good 2 core CPU? And therefore combine the pros of both: the faster clock rate of core 2 duo and i7's 4 cores.

Combine theese pros with the lower power usage of the Nehalem microarchitecture in i7. The i7 looks like the superior choice.

But will the i7 perform as good as a core 2 duo when it comes down to tasks that dont demand more than 2 cores? Intel lists the i7s with max turbo clock rate when just one core is active, but how should this be compared to the core 2 rate which is listed for 2 cores? If the base clock rate ("max" rate when all 4 cores are in use) for i7 is close to the core 2 rate, like 2,13 i7 and 2,53 core 2, I guess one may safly conclude that the i7 is better? But what about bigger differences? Like 1,6 i7 and 2,53 2 core? Which of these will be the best solution to a problem that just demands 2 cores?

The fact that all 2,53 core 2 arent eaqually good, just as there are differences between i7s with the same clock rate, makes the picture even more complicated. But are there som basic guidlines to follow?