In the Dell GX280, the memory modules are located near the processor as shown in the image below. There are three versions of the GX280 model. One is with a laptop-type DVD drive and is the smallest. Its memory has just 2 slots. The next one (whose picture is shown below) has 4 memory module slots and it has a desktop-sized DVD drive. The biggest one has PCI slots on the motherboard and has 4 memory slots.

I have checked the system information with SIW and have come to know that my GX280 has a DDR2 type memory which is PC2-6400. The image below has all information you need to upgrade Dell GX280 memory.

To open a GX280 casing, you will need to push 2 buttons on each side of the computer case simultaneously and pull the cover up.