Which form factor of the OptiPlex GX280 do you have? Small form-factor, small desktop, desktop, small mini-tower or mini-tower? The small form-factor has a maximum limit of 2GB and the rest have a maximum limit of 4GB.
What CPU model is installed in this system?
How many memory slots are currently being used? How many are not being used (i.e. are empty)?
What is the memory timings and voltage of the current memory modules installed on your motherboard?