I currently have about 8GB of RAM, and it's almost time to build a  new PC, and I was wondering if 16GB is overkill if I want to also play games like GTA V, which can barely run on my machine which is only three, or four of years old.  I took a look at what GTA V uses and it seems to be mostly GPU.  I seem to remember that FSX or FSX-SE (possibly too though I haven't checked, and can't anymore at present check) seemed to heavily draw on RAM, and processor.  Is this still true with FSX-SE?  FSX is not the only thing I will be using that requires a lot of juice, though it is one of the primary things I am thinking of.

8Gb is sufficient unless you keep many apps open during play or indulge in image manipulation/video rendering or similar.  Most users, I daresay, go for 16Gb 'in case.' However I would select 32Gb since then you could use an appropriate sized Ram Disc!