Windows 7  32bit  doesn't allow you to use past that amount of ram you see now. Either install win7 64bit or win10 x64
Hello,   I have an issue on a Precision T7910 : Windows 7 (32 bits version) displays in control panel > system that the pc has 64Gb of RAM but that only 3,17Gb are usable. The bios displays the 64GB of available ram, the memory tests of Dell Diagnostics do not show any error, everything is ok except a bus problem (ePSA 4236.32 - Warning. PCIe - Training error PCI tag-0400 VendorID-1093 DeviceID-C4C4 Svid-1093 SDid-742F Bus 04 : link degraded, maxWidth x4, negWidth X1, Slot4). Do you know where this problem of unavailable memory comes from and how to solve it please ? 
Precision T7910 : only 3,17Gb of RAM usable instead of 64Gb detected in BIOS