You purchased ECC memory (REG: registered)which is for server. It is incompatible with desktop pc such as 790.  Return those and get non-ECC desktop only memory pc3-12800U or ddr3-1600.  You can also use pc3-10600U or ddr3-1333, again make sure seller says desktop memory, not ECC, not registered or buffered.
The original PC came w/only 4G total.I Just purchased two Mushkin 8G DDR3s for my Optiplex 790, see attached photo. The PC uppon power up it beeps once, then short pause and beeps 3 more times, then pause and two more times, then shuts off. As my research shows, this is a memory error, "DIMMs not being properly identified or used". I since then, I removed CMOS battery, unplugged PC, held power down for 30+ seconds, put all back, still same. I put back the original memory, all is well! Any ideas? Aren't these compatible? As per my research, I can have up to 16G. Thank you in advance for ANY help! [IMAGE]
Optiplex 790 Memory upgrade