Besides, You also should care TOLUD(Top Of Lower Usable Dram) problem, when your rig has less main memories. This problem occur due to insufficient memory address space for accessing PCIE devices.
You can try reduce TOLUD value in BIOS menus ("Auto" does not help you), If there is such item. or You can try disabling "some unnecessary PCIE devices (like HDMI audio on video cards)" in device manager.