First of all the motherboard i m gonna work on is ASUS P8P67 Pro...i would like to know what happens when u get a ram-kit of let's say 2000 MHz and your motherboard doesn't "support" (at least doesn't refer to be precise) the EXACT MHz on the slots...instead it might support for example the 2133 the 1866..1600 etc. etc. what happens then? will there be a problem? can u and should u actually: overclock the ram? overclock the motherboard slots? (or underclock them?)
Also why do many motherboards refer to some slot speeds as (OC) ? Does it mean that the only way to reach that speed is by overclocking the memory slot or the ram? and why do also some motherboards have this "odd" feature -> example: "Due to CPU behavior, DDR3 2200/2000/1800 MHz memory module will run at DDR3 2133/1866/1600 MHz frequency as default" ? Does this mean u have to configure the speeds to the 2200/2000/1800 via the motherboard's BIOS? or it actually means via overclocking? (or some sort of a combo whatsoever)
My choice of RAM-kit is Corsair Dominator (the 2133 MHz 2x2 kit). What do you think of my choice (for 99% gaming purposes). Also should i buy the kit along with the Fans? does it suffiecently help with heat due to overclocking? (i m willing to pay for the fans if it does help, no prob)
No one can accurately answer the last question without knowing your MB model.
There is usually no problem using different speed RAMs. As long as the voltage, pins, DDR spec (...2, 3) are met. Don't fret the "EXACT" speed figures, it's just a disclaimer that if the CPU FSB is not what is expected then the RAM's speed will not be either. The RAM's programmed SPD will set the sticks to default. If you don't like the default, you can set them manually in BIOS. If you exceed the stated spec, then you are OCing them.
You can overclock but you don't gain much doing it. You can underclock (I do to save a few pennies on electricity), I run my RAM at 1.34v.
OC speeds means that your motherboard won't automatically set your RAM to that speed even if the SPD (info contained on the RAM itself) says it can operate at that speed, it will default to the next lowest. It means is that the board is capable of making the RAM go at that 'OC' speed - but you will have to set the options manually to configure it to do so.
The bit about cpu behaviour basically is down to the fact that the RAM speed is determined by internal clocks within the CPU, BCLOCK (or Northbridge FSB in the previous gen.) Your RAM speed is a created from a multiple of different numbers which may not always come out at the neat rounded speeds the manufacturer quotes - In practice these differences mean very little!
The RAM speeds are rated at the maximum that the manufacturer will guarantee the RAM to go at. Often RAM spec'd at a lower speed will operate at a higher speed with relaxed timings. (Not having played much with DDR3 - the best example I have is the 'performance' DDR2-800Mhz kits that were specified at 800Mhz with 4-4-4-12 timings, these would nearly always operate at 1066Mhz with 5-5-5-15 timings). The best RAM will have the lowest timings at the specified speed.
In practice a decent 1600/1800Mhz kit will be plenty good enough, but if the 2000 kits are cheap enough then why not.