Optiplex 390 Tech Guide 

NOTE: Memory modules should be installed in pairs of matched memory size, speed, and technology. If the memory modules are not installed in matched pairs, the computer will continue to operate, but with a slight reduction in performance. The entire 8GB memory range is available to 64-bit operating systems. MT DT SFF Type: DDR3 Synch DRAM Non-ECC Memory 1333MHz DIMM Slots 2 2 2 DIMM Capacities Up to 4GB Up to 4GB Up to 4GB Minimum Memory 1GB 1GB 1GB Maximum System Memory 8GB1 8GB1 8GB1 Memory configurations 8GB1 DDR3, 1333MHz, (2 DIMM) X X X 4GB1 DDR3, 1333MHz, (1 DIMM) X X X 3GB DDR3, 1333MHz, (2 DIMM) X X X 2GB DDR3, 1333MHz, (1 DIMM) X X X 1GB DDR3, 1333MHz, (1 DIMM) X X X

because 390 has only two DIMM slots, you can install either identical spec ram or different capacity of ram of same speed.  It is considered by human user odd to run two rams of different speed but the pc does not care and it would default to run at the slower ram speed.