I have the same problem on my Dell Precision 370. I have 4GB ram but only 3GB is usable. The rest is used by Win 7 X64. I'm about to treat myself to 8GB for Christmas.
My Computer
Computer type
PC/Desktop
Computer Manufacturer/Model Number
Lenovo IdeaCenter 450
OS
Windows 10 Pro X64
CPU
Intel Quad Core i7-4770 @ 3.4Ghz
Memory
16.0GB PC3-12800 DDR3 SDRAM 1600 MHz
Graphics Card(s)
Intel Integrated HD Graphics
Sound Card
Realtek HD Audio
Monitor(s) Displays
HP 22" LCD
Screen Resolution
1680 x 1050
Hard Drives
250GB Samsung EVO SATA-3 SSD
2TB Seagate ST2000DM001 SATA-2
1.5TB Seagate ST3150041AS SATA