Here is my observation from going from 4GB to 8GB. Been sitting on 8GB for some time now.
With 4GB I was using 50~80% regularly. With the move to 8GB, I use 50~75% regularly. Using 68% right now. My usage in part is a little higher now due to the fact that I am running a few more processes (would have run anyways on 4GB) but also that I don't close out as many windows/programs now. On 4GB I would sorta "clean up" my usage just to keep some memory free but now I don't have to do this.
I could totally and easily use 16GB. Im sure within a couple of weeks my usage would be at and over 8GB.
I'd like to see the real-world performance differences between 1600 MHz CAS 9 and CAS 7 on Sandy Bridge.
Can Sandy even do CAS7? Haven't seen a set that could TBFH well maybe the Dom GT's but not many people can afford them. SB's mem controller is pretty weak compared to high end Inter chips. My CAS6 sticks needed to be set to CAS8 @ 1600 to be stable and all the "latest and greatest" Ripjaws and whatnot are speced at some super crappy CAS10-12-12-27 or such. CAS9 on Sandy is considered good.
The best thing going from 4 GB to 16 GB is being able to Alt-Tab from games to other apps without the system hanging as it rapidly pages. Useful when I have spotify on in the background or need to look something up on the web and don't want to exit and reload the game. That being said, resource monitor only has me using 12% of my RAM as only a browser is currently open.
Good call getting the mem now as the Elpida bankruptcy could make prices increase (as I posted in the Other Hardware forum).