Does unsupported RAM effect display cards? I have a MacBook Pro 17" Mid 2010 which normally supports 1067 MHz RAM. I needed more memory and installed 2 x 4 GB RAM 1333 MHz modules which - as expected -  also work with 1067 MHz.

After a couple of weeks the display started flickering and showed disturbances similar to television statics for a second and then returns to normal. A couple of weeks later this happens more frequently like every 5-9 minutes. It especially occurs when a secondary display is attached.

Could the RAM modules cause this or is my display card just dying?



This is how it looks like: video
Yes. Apple uses some of your RAM as VRAM as explained in this answer.

So using unsupported RAM can affect your integrated video card.  Try switching back to the stock modules that came with the device and see if you can still replicate your problem.