To be perfectly honest the levels of ram you are currently dealing with are only useful for massive projects.
Dreamworks /pixar level stuff, that requires a render farm to actually resolve.
What's the largest thing you've tried to manipulate in photoshop?
If you are using a recent version it is not shy at all about consuming all the ram it can find , when it actually needs it.
If you are dealing in images or small medium projects not exceeding typical 1080p or so resolutions.
These things do not require massive amounts of ram to process.
A system with 8 gigs of ram could easily handle the job 90% of the time.
I only have 16 myself and with gaming, digital editing, and all the stuff I ever do on this system I never consume more than about 6 of it. This isn't because windows is being stubborn and not using the memory, it's because it is using the memory, and that's really all of it needed.
To be perfectly honest with you I consider myself to have personally gone full overkill just getting 16gigs.
There is also the problem that the more stuff you keep active and floating in ram the higher the probability of processes going rogue/stopping under a fault, and the system becoming unstable.
This is the reason most applications and even windows itself somewhat limit their footprints.
If you really insist on making photoshop flood your ram as much as it possibly can, there are actually some settings to make it do that though.
Optimize performance | Photoshop CS4, CS5, CS6
Root around through there for a bit.