Why not all the RAM in use?

My guess is that portion of your RAM is probably being used as video card memory.

What video card / motherboard does your computer have --- or what model is the computer?

Yes, it appears you are using a video card (integrated most likely) that is sharing the memory.

Many Integrated cards do this, and some discrete such as the ATI x1300 share memory and take their up to whatever the card is rated for from the system memory. So a card with 512MB shared may take up to 512MB from system RAM.

If you post your GPU and motherboard we can confirm

The amount of reported RAM will be the amount of physical RAM minus the amount of physical address space reserved for the other PCI devices. Enter the BIOS and see if there's a memory remap feature.

If it is not in use by you videocard, then try to set MEMORY REMAPPING to ENABLED in BIOS. If available.
What I did to check if there isn't something wrong with the hardware, I downloaded an ISO from Ubuntu and booted the CD and let Ubuntu Start and checked if it uses the full 4gb. And it did.

As posters before me mention... Onboard GPU's do this.  The GPU itself may be set in the bios to use 64MB or 128MB or even more, but some use discreet memory to steal memory from teh system when it needs to.
Some utilities like nVidia and the AMD (nee ATi) CCC can adjust the amount it uses.

"... The amount of reported RAM will be the amount of physical RAM minus the amount of physical address space reserved for the other PCI devices. "   ==>  WRONG.    This would be true for a 32-bit OS, but not for a 64-bit OS -- in a 64-bit system the reserved memory for PCI addressing, BIOS shadowing, Video addressing, etc. is FAR above where any physical RAM will reside.

The issue here is most likely that a video card that uses shared RAM has been assigned 750MB of memory.    To tell for sure, look at Device Manager - View - Resources by Type - Memory

Go to Run and type msconfig and click ok. in the opened window  go to the boot tab and click advanced options on the boot you will find two options, number of processors and max memory. Select the box max memory and it should automatically put the amount of memory installed. Once you reboot it will allocate more memory to your system but still hold a bit back for hardware requirements.

Thanks everyone.

It is a Dell Precision 370 serial GSVNB71.
There is no output for video on motherboard. The videocard is ATI FireGL V3100
I cannot find a driver at AMD's site for this card.  Using the MS driver.  Aero performs nicely BTW.
2 sticks of 2GB RAM of Crucial RAM scan recommened recently installed.  In correct slots. 2 of 4 slots are filled.

Latest A08 BIOS has been running for a month.  I don't see CMOS settings to manipulate any kind of memory setting.  I've looked through every option.

I booted to latest Ubuntu LTS CD. It also only shows 3.2 RAM. (screenshot).  It is probably 32bit, and I'm not sure how that might be significant.

garycase, not sure what to look in device manager.  here is a screenshot.


Screenshot-System-Monitor.png
DM-mem.png

Keep in mind that this system only supports a max of 4GB of RAM, so Windows is likely hitting a "ceiling" for system resources with which to manage hardware devices.

Crucial says:

Maximum Memory Capacity:  8192MB
Currently Installed Memory:  4GB
Available Memory Slots:  2
Total Memory Slots:  4
Dual Channel Support:   No
CPU Manufacturer:  GenuineIntel
CPU Family:  Intel(R) Pentium(R) 4 CPU 3.40GHz Model 4, Stepping 3
CPU Speed:  3391 MHz
The ECC and non-ECC modules cannot be mixed.Installed in pairs of modules.

I'm not expert at these things, but maybe, pcrequest can use physical address extention? Does anyone know?
With: " bcdedit /set [{ID}] pae ForceEnable "

PAE is only for 32-bit OS's ... it allows Windows to simulate a 36-bit address pool.  Windows 7x64 is already using a 64-bit.

@PowerEdgeTech: Thanks. So No PAE for pcrequest. It was just a thought.