question regarding usage of memory

I have got Quadra core AMD machine having 4 Gb RAM.
The problem is when I install 64-bit Windows 7 I get frequently blue screens.
With 32-bit Windows 7 there is no problem.
But it shows only 3 gb RAM as a usable.
so one GB RAM is lying and used...

Besides 128 MB RAM is used for shared Nvidia graphics card which is on board.

My question is whether this 120 mb ram that is being used for graphics card is taken from the other one GB Ram (which is lying unusable with the operating system) or this 128 MB RAM is used from within the three Gb's that is used by the operating system.

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