Dell Optiplex 780 Small Form Factor (SFF) - What type of RAM I must use to upgrade to 8 GB successfully ?

Hi ,


I try to upgrade the RAM further to 8 GB but was unsuccessful.

I tried to used Kingston DDR3 1333 Mhz 4 GB , and DDR3 1600 Mhz 4 GB,
both the PC responded with beeps indicating that the RAM is not
suitable. Under the specification it says DDR3 SDRAM (Non ECC Type) Speed 1067 Mhz is the type

I notice that my vendor upgraded to 4 GB wsuccessfully  using DDR3 1333 Mhz 2 GB RAM at each slot,

Is Dell Optiplex 780 SFF only support 2 GB RAM at each
slot and not anything more than that ?

What type of RAM (model / part number) I should used from the market that could make it work.

Appreciate if you could provide useful advise.

Thanks

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