RAM quandry DDR3

Right. Currently run HyperX savage at 2400. I have 2 queries:

1) I'm seeing committed memory at 9/16gb when playing games. I have steam, AB and avg running. No chrome. BUT actual ram says 6.5/8.. And yet I see constant c drive access when playing Witcher 3 for example. WHY?

( and some hitching when entering new areas, but locket at 60fps usually)

2) if I add the same ram from a different kit (so would have 4x4gb) is it very unlikely it'll not run at 2400? I know this is an uncertainty...Bear in mind I dont have the greatest 3570k needing decent volts even for 4.3, and am pulling imc of 1.05 to hold 2400 cl10 as it is. BUT am rock solid stable (prime, memtest, gaming)

Help appreciated. Going 6600k later this year so don't want to invest in £80 of 16gb from one kit if not needed.
 
Solution
well witcher 3 is a demanding game far as i remember

but if in the uk guess you could buy the ram online and if not happy you have 14 days to return it any way
Thanks mcnumpty. Yeah, I understand it'll help itself. I guess I'm wondering why it uses pagefile when I have 1.5gb spare ram according to task.manager.

And also if I drop 35£ on another 2400 8gb kit, is it likely to hit that on an ivy z77 platform...given my current ability to run it with increased volts.
 
i skipped z77 went from z68 to z170 so no idea about z77 boards

but personally think you will be fine with 16gb and adding ram is never 100% certain to work even if you get the same make and model number

i would stick as you are and save the money for 6600k--which is what i got