Keep in mind that windows may not use all of your ram even if you have tons of it. It likes to put things in its cache (swap file). By default I believe it creates the swap file as 1.5x your amount of ram. So even if you have 12GB worth of programs running it may put most of it in ram and put some of it in its cache (prob why you saw it using 9GB ). I think the only way to force it to use all ram is to disable the swap file but most people suggest not doing that since it can cause issues. With 24GB though you may not encounter any issues.

Also, If you do get the 24GB of ram make sure to check the motherboard manufacturer's website to make sure that it is compatible with whatever ram you get. That may actually limit what specific kits you can buy. I had an issue with mine where I could put in 3 sticks of ram with no problems but had lots of issues when I fully populated all 6. I ended up having to get a different pair of kits that were compatible based on the vendor list for the motherboard. Not sure if this will be an issue for you but figured I would mention it in case it is.