Desktop replacement: dual or quad core?

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I am planning to buy a new laptop this weekend, and I want a desktop replacement - just tired of small screens and low performance, and, unfortunately, I simply do not have a desk to put a high-end desktop. If I was to buy a desktop, it would certainly be equipped with a Core i7-860 2.8 GHz processor. While I cannot replicate such power in a notebook, I am trapped in a dilemma of choosing between four cores or a high clock speed. In a desktop, it seems to me that such dilemma does not exist, since the processors can use more power and have higher clock speeds: it would be worth more to have a 2.66 GHz or a 2.8 GHz four-core than a 3.2 GHz or a 3.33 GHz dual-core. But in laptops, it's a completely different story.

The contenders are basically the Core i7-620m and the Core i7-720QM (or the i7-740QM), and the runner-up is the Core i7-820QM (and i7-840QM), if it is really worth it.

I have done extensive research on the Internet. The sources are not many, but it led to to some conclusions.

The Core i7-720QM 1.6 GHz has 4 cores and 8 threads, but a low clock speed (1.6 GHz) that can be increased up to 1.73 GHz with all cores operating. It can go as high as 2.4 GHz with only two cores. The Core i7-620M, on the other hand, has a clock speed of 2.66 GHz, which can be increased up to 3.06 GHz with Turbo Boost using all cores.

Theoretically, the i7-720QM would be a superior processor. Intel has put a higher number to identify it (7 instead of 6... duh...). Four cores operating at 1.6 GHz equals to 6.4 GHz (in the case of i7-720QM), while two cores operating at 2.66 GHz equals to only 5.33 GHz (in the case of 620M); with Turbo Boost, the i7-720QM would sum it up to 6.92 GHz and the i7-620M to 6.12 GHz - still a big gap in performance. In addition, the i7-720QM has 6 MB L3 cache while the i7-620M has 4 MB. But that's just theory. In real life, things are a little bit different.

I do have a Core 2 Duo right now and I see Windows making extensive use of the two cores. However, I suspect that even the most advanced software are not 100% optimized for multi-core and multi-threading processing. A clock speed of 1.6 GHz could be a disadvantage in this scenario, especially comparing to the much higher clock speed of the i7-620M. The i7-620M has a clock speed which may be 1.33 GHz higher than the Core i7-720QM, and that seems to me a big difference.

I've read that the i7-620M would be better for single-threaded or lightly multi-threaded applications, due to its higher clock speed. Some sources say that the i7-720QM would be the better choice for heavily multi-threaded apps, while others say that, due to the huge difference in clock speed, the i7-620M would be the winner even in those cases.

There are other factors which must be taken into consideration, of course. The i7-720QM has 6 MB of L3 cache, while the i7-620M has only 4 MB, and that makes a difference - however, I do not know whether this additional 2 MB make it up for the huge gap in clock speed. The i7-720QM also supports 1333 MHz memory, while the i7-620M supports only 1066 MHz memory, and I do not know how this will affect the performance - some sources even say this is irrelevant.

The i7-620M is a 32nm, 35W, processor, and consumes less power than the 45nm, 45W, i7-720QM. It means less heat and more battery life, of course. But, in terms of performance, less heat and less power consumption could mean that the i7-620M could achieve its top Turbo Boost performance more easily than the i7-720QM. As a result, it would be more likely for the less-power hungry i7-620M to achieve the 3.06 GHz Turbo Boost performance than the i7-720QM to achieve the 1.73 GHz.

I've seem some benchmarks on the Internet. In Passmark Software's website, the Core i7-720QM scores higher than the i7-620M, but I am afraid these numbers reflect a theoretical performance that nobody will ever get to achieve in day-to-day situations. Notebookcheck has done some comparison too and it ranks the i7-620M in a higher position than the i7-720QM. The i7-720QM scored higher than the i7-620M in most tests, but Notebookcheck thought the i7-620M would be a superior processor - perhaps it thought the i7-620M would be faster in real-life situations.

I have not yet come to a conclusion. I am leaning towards the i7-620M, as it would be a faster processor than the i7-720QM in most situations. I suspect I do not use very multi-threaded applications; however, I do extensive use of multi-tasking. It is very often to me to keep Microsoft Office applications, a web browser, a multimedia player, Adobe Reader and other applications opened simultaneosly. And I like them all to run very fast all the time...

Now that I have done this research and that I still can't reach a conclusion, I would like some help. Are the conclusions I came up with (above) correct? Which of these processors would be faster and which one would be the better choice? And how these processors would compare to mine (a Core 2 Duo T8300 2.4 GHz)? Thank you for the help!
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