•The Dell Optiplex GX520 & GX620 Series includes:
•Optiplex GX520SFF SD (Small Form Factor / Small Desktop)
•Optiplex GX520DT (Desk Top)
•Optiplex GX520MT (Mini Tower)
•Optiplex GX620SFF SD (Small Form Factor / Small Desktop)
•Optiplex GX620DT (DeskTop)
•Optiplex GX620MT(Mini Tower)
•Optiplex GX620 USSF(Ultra Small Form Factor)
If your not yet quite sure which system you have, here is :