Photos and screengrabs sometimes turn out a little blurry when uploaded to this site.  So I can't read the details on your RAM sticks. I can tell you some Dell's won't take mixed brand RAM.  Best bet is to get another stick of the A-Tech RAM, and all specs should match. I use A-Tech myself and have been happy with it.  While my 7010 has mixed brand RAM including A-Tech (I simply added RAM), there's non-Dell PC's that don't take it either. I ran A-Tech exclusively in my previous 755.  And why not?  It has a lifetime warranty.
I bought a refurbished Dell Optiplex 390 to run a small Minecraft server with and it came with 1 4GB stick of ram. Today I bought another 4GB stick of ram to use in the 2nd slot. The computer runs fine with either stick of ram in one slot, but becomes very unstable when using both. Both sticks of ram passed memtest86 in both slots. Any ideas as to why the computer becomes unstable (crashes, constantly restarts and fails to boot) when both sticks are installed? [IMAGE]  the larger blue stick is what came with the computer and the smaller green one was recently purchased.
Issue with installing more RAM dell optiplex 390