Dell computers are fussy about memory. That Amazon seller's ram might not work. Stick to Crucial for ram for Dell computers. The max ram is 16gb. Remember your model computer is 9 yrs old. That ram on Amazon is sold by unknown seller. Amazon is not the seller. Amazon is a marketplace now like Ebay.   
The system specs of my computer say MemoryType DDR3Speed 1333 MHzConnectorsDesktop, Mini-Tower, Small Form Factorfour DIMM slotsUltra Small Form Factor two DIMM slotsMemoryCapacity 1 GB, 2 GB, and 4 GBMinimum Memory 1 GBMaximum memoryDesktop, Mini-Tower, Small Form Factor16 GBUltra Small Form Factor 8 GB   Mine is small form factor not ultra small form and I want to buy four of these: [URL]   is this compatible?
optiplex 790 sff ram question