See
RAM, storage devices and a reasonable amount of low power accessories are almost irrelevant (for overall power consumption) if you're buying a decent (and large, obviously this wouldn't apply to a <=150w mITX HTPC) power supply and not trying to run it at 90%+ capacity. The problem is that it stresses the memory controller in the CPU or NB on pre-Nehalem Intel CPUs (not sure when AMD started using on CPU IMCs) which can result in worse performance. If you're running at stock or lower there should be no problems but if you're running above the specs you may find yourself having more trouble with extra DIMMs.
Have 48GB of ram, enjoy the ability to run 5-6 Different games at once, and never need to close them down.
It's not too much if you happen to run Vista lololol
With the current price of DDR 3 RAM 12gb is more then justified. It will cost you the same to get 6gb at this point.
And who knows maybe you decide to use your PC for things outside of gaming.
I got a 3ds Max file that on a 16gb system used 15gb for Max alone. That was just the loaded scene. It would use even more if I was going to render it out in a single pass.
High poly sculpting in Mudbox also uses memory like crazy.
I'm building a new rig and ended up going with 16GB (4x4 ddr3-1600) since memory prices are dirt cheap right now (16gb was $70)