It was a bit tricky finding information since LSHW was in french. However the usermanual for the model seems to cover it
"For optimal performance fully buffered DIMMs (FBDs) should be installed in matched sets of four. This enables quad-channel operation and provides the highest memory bandwidth. The sets of four must be matched in size, rank, and organization. This information can normally be found on the FBD label. For example, the label of each FBD in a set of four might read: 1G 2R x8, in which 1G represents the size of the memory module, 2R represents the number of ranks, and x8 represents the organization."
This means you should be able to install a set of 4 memory modules of 2gb with no issue alongside your current ram. You can find the manuals here