I would like to know how to think or rather how to interpret quality of a CPU. For instance, for operations such as rendering, what works better - a CPU with a greater number of cores but less capacity of each of the cores, or a CPU with lesser number of cores but with more capacity of each core? What does it all depend on? How could I infer if one CPU would work better or faster than other if they have different amount of cores and different capacities on each of them?
Different CPU's have different strong points and weak points. Higher clock speeds tend to favor higher performance in single threaded applications regardless of core count, but not always. Internal micro-architecture can dramatically effect performance of two different CPU's at the same clock speed due to how efficient some processor resources are. And on top of it all the optimization of the application itself can do wonders. It has been found for example that the Intel compiler, the one given to pretty much everyone in the industry strongly favors Intel CPU's and has been known to compile code that gimps itself on AMD / VIA CPU's. Also multiple core processors don't shine until you start multitasking. A benchmark program won't be able to replicate the load that having FF open with 20 tabs, some background downloading while your playing a FPS shooter will place on your system.
The benefit of multiple cores really depends on the apps you are running and how you are running them. In tom's various tests it seems that rendering and video processing are functions that can make use of multiple cores and sometimes 4 cores with lower clock speed will beat 2 cores with higher clock speed. This doesn't mean that all apps that render will do this or that all apps the process video will do this. You have to check to see what your apps will make use of.
And then it depends on how you are running them. If you are doing a virus scan in one window, downloading files in another window, processing video in a third window, and checking email in a fourth window, then you can make use of a lot of cores. But if all you do is to have a big number of tabs open in Firefox then it really doesn't matter, because only the tab that you are viewing is actually doing something, and one core is fine for you.
Every day I use the aforementioned core 2 duo machine at work and the core 2 quad machine at home. I can frequently be copying files, plotting drawings, and other work at the same time and bring the dual core machine down to the point that it is hard to use. I have never been able to do that with the quad core machine. It will crank up a virus scan in the background and I can never tell when it starts and when it finishes. The machine just keeps running without a slowdown.
Also as mentioned you have to consider the efficiency of the microarchitecture of each different cpu when comparing them. For instance I built a machine with an overclocked Intel core 2 duo processor and another one with an overclocked Intel core 2 quad processor. Then later on I built a couple of machines with i5-750's. In the process of overclocking and testing the i5-750 I set it to the same clock speed as the previous core 2 quad. Running a simple program like superpi the i5-750 was 15% faster, at equal clock rates. So intel has increased the efficiency of their microarchitecture. Now that we have the next generation of Intel cpu's such as the i7-2600, its microarchitecture is more efficient than the previous generation such as the i7-920. The newer generations can run at faster clock speeds too so they perform better in 2 different ways. AMD chips are entirely different animals with their own microarchitecture so to compare then with Intel you have to compare benchmark results and not just clock speeds. This can be hard to do but in general you can look at various tom's systembuilder marathons and compare the benchmark results for the intel chips that they run with the AMD chips that they run, and do the math to adjust the results for different clock rates.