Here's how I have been comparing processors:
Intel Core2 Duo P7450 (2 cores, 2 threads) @ 2.13GHz PassMark => 1572 , passmark/thread => 786
Intel Core i7-2620M @ 2.70GHz (4 cores, 8 threads) PassMark => 3415 => 854
On tasks that can use all cores, the i7 will provide a bit more than double throughput. For single threaded tasks, it will be less than 10% faster. Hyperthreading will provide some additional performance boost when there are more than 4 tasks competing for CPU cores.
The 2720qm was not listed at cpubenchmark. It has a higher clock rate and can use faster RAM, but I don't know how much either will contribute to raw performance.
This analysis also ignores any benefit you might gain from overclocking.
I'm curious to know if this is a reasonable way to compare two CPUs. I see some reviews that compare multiple benchmarks and ranking is rarely the same across all processors tested. However, if both processors are not included in the same test, I'm not sure results from different tests (under different conditions) can be compared.
(PassMark from http://www.cpubenchmark.net/cpu_list.php)