i have 2x4gb 2666mhz sticks on my 4-ram-slot motherboard, and i want to add another 2x2gb 2666mhz to get 12gb of ram... will that work or do i need to get the same capacity (another 2x4gb 2666mhz kit) ?
i only game and i dont need 16gb of ram if it can be avoided
While it may work, I can't guarantee favorable results.
Here's the thing, even two of the same exact kits of memory may not get along with each other. The only way to guarantee that it'll work not just enough to boot, but also well enough to be error free and not cause problems would be to buy a kit of memory that has the capacity you want.
In your case, since you want more than 8GB, you'd want a 16GB kit. I mean, some manufacturers offer 12GB kits, but those are usually single channel because it's an odd number of sticks. 16GB would be your best bet. Either 2x8GB or 4x4GB would work for you.