I manage to figure it out. I know it might be a common thing but I will still write my solution here just in case. The problem was with Hardware Reserved from Windows. which took like 4.6GB. To fix that go to Run(windows logo key + R) and type in msconfig then enter. In the Boot tab click Advance Options and deselect Maximum memory. Restart and now my computer is showing the whole 8Gb RAM. Good Luck, Mircea 
Hi there, I have installed today another 4Gb Ram in addition to the original 4Gb that it already had. However It says Installed memory (RAM): 8.00 GB (3.48 GB usable) System type: 64-bit Operating System Processor: Intel(R) Core(TM) i5-3450 CPU [NAME].10Ghz 3.10Ghz Manufacturer: Dell Model: XPS 8500  I had a look in bios as well and I can't see any setting regarding memory. 3.48 Gb usable it was showing also when I had only 4Gb installed and I went with the assumption that half of gig is being used by the video card although is supposed to have 1Gb dedicated memory. According to dxdiag my video card is Nvidia GeForce GT 620. My question is how can I have the windows to recognize and use all 8Gb of RAM. P.S. Can a moderator move my post to -> Desktop General Hardware Forum Thank You
Installer Ram memory On XPS 8500 64-Bit