HELP with RAM upgrade

If you install RAM with different clock speeds, all of them will run on the lowest (AFAIK). Mixing RAM is never a good idea but unfortunately I can't help you on this one.

Easy just http://downloadmoreram.com/

How do you plan to install 3 more modules if your cooler doesn't let you now? It will work, but put modules of same speed and voltage if you can.

Mixing RAM is never a good idea. It can work, but it may be slow as fuck, as the memory controller has to work around all the different timings. (Normally it uses the worst timing on everything, but it still can cause a feel of being slow.)
The bare minimum to look for is to have the same voltage and latency. Frequency is a lesser issue, since they will default to the lowest clock rate, which is not such a huge problem.
In any case, if you expand and not replace the RAM sticks, expect the memory controller to be a little grumpy from all the extra work. :)

but lower Volt at 1.5V.

You'll have to undervolt the other RAM yourself in the BIOS (unless your mobo does it automatically) in addition to what Aseed said (the RAM will run at the speed of the slowest stick). My advice, to avoid any potential problems, would be to just get a new RAM kit (all the same brand/speed/timing). If you can't get a stick into the #1 slot on your motherboard, a 3x4GB kit might be your only solution anyway. :X

Never a good Idea to mix RAM, especially if the clocks, CAS, voltage is different. You are better of buying a complete new set, or risk crashes, BSOD and system instability.

Using a low profile heat sink ram will allow you to install under the 212 EVO, you can also mount the fan slightly higher to compensate if needed.

Thank you guys for your input.

I think I'll just buy a new 3x4Gb kit and get done with it.

is this G.SKILL kit going to work with my mobo, since it's not listed in the compatibility list of the manufacturer.