Why is it only 3 GB usable?  

The system is Dell Optiplex 330, the RAM in the system configuration says only 3 GB is usable, the installed RAM is 4 GB. Is there a solution to this?

Muz said:

The system is Dell Optiplex 330, the RAM in the system configuration says only 3 GB is usable, the installed RAM is 4 GB. Is there a solution to this?

32-bit or 64-bit Windows 10? If 32-bit, then that is normal. 32-bit address space is a maximum of 4GB, and about 1GB will be 'hardware reserved' for use by the video adapter.

All 32 bit Windows only use 4 GB of capacity? Graphic card is NVIDIA Geforce 9500 GT. How do I know this graphic card is using the 1 GB?

Hi,
32 bit is often less than 4gb's
My 32 bit has 3gb's another one only has 2gb's

Just depends on the manufactures hardware used.

Muz said:

All 32 bit Windows only use 4 GB of capacity?

Correct. The total address space x86 Windows can use is 4GB, so the video RAM has to occupy some of that. A clean install of x64 would be able to map the video RAM above the 4GB installed RAM, using only about 100MB as Hardware reserved.

Graphic card is NVIDIA Geforce 9500 GT. How do I know this graphic card is using the 1 GB?

Run the System Information app. Look under Components > Display. Also look at Hardware Resources > Memory.

See System Information in Windows 10

Unless you are using Windows Server 2003 enterprise x86 which can support 64gb ram

SIW2 said:

Unless you are using Windows Server 2003 enterprise x86 which can support 64gb ram

That is true, but perhaps a little unlikely in the OP's case.