The more RAM being used the better. 7 will unload and load things as needed if available RAM becomes a issue.
The largest bottleneck in any system is the traditional mechanical drive. Pagefile works similar to RAM only its reading and writing to the HD rather than the RAM.
This method is extremely slow, and best avoided (which it will on its own) if you have the available RAM for 7 to use.
Even if you have a SSD, while much faster than spinning drives, is still no where near as fast as RAM. So it still holds true.
This is why Superfetch, Pre-Fetch are still viable, be it mechanical or SSD drives, as it keeps things in memory.
Bottom line, let 7 use all the RAM it wants too and do not mess with it. Your system will run better in the long run.
Note ~ Thats not saying disable the PF. it is useful. You just dont want to access it unless you must.
--The RAM decrease your seeing may also simply be put aside into Standby Mode.
Standby: Items loaded into memory but not currently active..
It quite simply means, its in memory but not activly being used. But is ready to be marked as 'In use" status when needed.
In use: is what it says, whats actually being used currently.
Look in resource manager to see. you may find you have X amount in use, x amount in standby and 0 RAM available.
This is a good thing, as it means 7 is actually taking advantage of, and using your memory.