I've got 12 gB of RAM also - so running Memtest overnight is a requirement.
Make sure that it's completed 3 passes (when I did it I only got 5 passes overnight)
It's my opinion that 6gB of the RAM is wasted (I've monitored the RAM usage on my system) - but since you've already purchased it you might as well leave it in. When I build the wife's next system, she'll get 6 gB of my RAM. There's just nothing that I do that requires all that memory.