Looking at the specs for Optiplex units that are available for sale, that model has an Intel 2xxx CPU. That would require regular 1.5V DDR3 memory.
According to this link -- https://www.dell.com/downloads/global/products/optix/en/optiplex-390-tech-guide.pdf Maximum RAM is 8GB (2 x 4GB).
According to this link -- https://www.dell.com/downloads/global/products/optix/en/optiplex-390-tech-guide.pdf Maximum RAM is 8GB (2 x 4GB).