10gb Intel nic on Optiplex 3020 SFF removes one memory bank.

Its related to how the PCI-E card is addressed in the memory map.  Base Address Registers (BAR) in the PCIE configuration space hold the memory address that the PCI Express should respond to / is allowed to write to. 

PCI-E BASE ADDRESS MEMORY

It is most likely related to BIOS installing NIC shouldn't disable a RAM module. Speedstep answer is not valid in this case it is only related to adressing the NIC.

Base address Registers (or BARs) can be used to hold memory addresses used by the device, or offsets for port addresses.  Typically, memory address BARs need to be located in physical ram. The Type field of the Memory Space BAR Layout specifies the size of the base register and where in memory it can be mapped. If it has a value of 0x00 then the base register is 32-bits wide and can be mapped in the first 4 to 8 gigs.  The card he is using is an X4 or X8 card and he is using it in an X16 video slot.

The motherboard does not map that slot for use with NIC or storage cards.

Sorry, speedstep you're incorrect. I ran into this issue today with an Optiplex 3020 SFF and an Intel i350-T4 NIC. I lost an entire memory slot upon installing the NIC in the x16 slot - same as the OP. Fixed it by following this video/guide How To Stop Your HBA From Disabling Memory Channels I now have the Intel NIC installed and all installed memory (both slots) available. Others have had this issue on various motherboards from various manufacturers with many HBA's and a few NIC's and they have all had success with this peculiar loss of a memory channel.

I know this thread is over a year old at this point but this information can definitely help someone like it did me.

@PClarke313​ This video is what solved my problem as well. I taped over pins 5 & 6 on the bottom side of the NIC and now I have all of the memory slots back. Thanks for pointing in the right direction.