memory - Where does Windows 7 waste my RAM?

Short version:

I have a system with 4GB of RAM, and another one with 8GB. Both running Windows 7, with paging file disabled. Now the strange part is that I can open the same number of programs before Windows 7 says there is no more memory for new programs. I would expect that with double the memory I could open more programs before running out of RAM.

Long version:

Something is not adding up to me. A few years ago I had a 32-bit system, running Windows 7, with 3 or 4 GB of RAM. I had turned off the pagingfile, because the system is faster that way.

Back than I could have open an instance of Visual Studio, MSSQL Management Studio, a couple of Firefox tabs, and a couple of Chrome tabs. These were the ones that fit in that 3 or 4 GB RAM.

Now I have a 64-bit system, running Windows 7, with 8 GB RAM, pagingfile turned off.

I can have open the same number of programs: an instance of Visual Studio, MSSQL Management Studio, a couple of Firefox tabs, and a couple of Chrome tabs.

Now my question is, how does this add up? I cannot really feel the difference between 4 GB or 8 GB of RAM. I've been thinking of buying more RAM, but the motherboard does not support more. But even than, I'm not sure It would make a difference, since it seems the RAM is wasted somewhere.

What can I do to identify where the RAM is allocated, and more importantly, what can I do to keep the RAM unallocated/so I can use it for my purposes, not for who knows what hidden Windows features.

EDIT:

I have turned off paging, because writing to the disk slows the overall performance, and I think that amount of RAM should be enough to live with.

I think there is a problem here, because if I open some other programs on my 8 GB system, than windows alerts me that there is not enough memory, and I should close some programs.