I was wondering if I could leave one older 2 GB RAM module in so my three channels would be atleast partially filled up with 2 x 4GB and 1 x 2GB.
From what I know the speed of your RAM is based on the slowest module, not on the fastest. So my real question is, what is more effective: to fill in 2 channels with just the 2 x 4GB or the fill up all of the three channels with 2 x 4GB and 1 (slower) x 2GB ?
It is better to have the 2 cards installed because both are 4gB each. If you set up a configuration with two 4gB cards and one 2gB card the triple channel setup then will not work at full speed. I'm not just saying just that the config will then work at 1333MHz, other speed related issues occur. If it were a 4gB card then there wouldnot be any issues, just that the other cards would be dragged down to 1333MHz and nothing else. Also using different cards will make your system to crash randomly.
According to me your old setup is much better than your new setup that is if you do not need that extra 2gB of memory. Triple channel 6gB is much better than dual channel 8gB.
The problem is I want to run planetside 2 and with my current setup I'm spiking up to 80% ram usage which obviously causes massive lagg. That's why I initially wanted to upgrade my memory. So the best solution here is to buy another 4GB card right, because going back to 6GB is not going to solve my problem. And thanks for warning me before I even tried the 2 x 4 gb and 1 x 2gb setup, random system crashes don't seem very pleasant.