At least MB's for a 790 are a bargain.  Remember to include the 790's size in your search term. [IMAGE]  
I have a Dell Optiplex 790 and both 1 and 3 don't work but 2 and 4 are fine and I don't know why, to be clear I'm using A-Tech 4GB DDR3 1333MHz DIMM PC3-10600 UDIMM Non-ECC CL9 240-Pin because it didn't have any ram installed and I only have this single stick nothing else. Details that could help find the problem: 1) The 790 is a used computer by someone one who no longer needed it so they or that single person could have done something that I'm not aware of and I did do a reset so I can make it my own so maybe something was changed or was changed before the reset and that change is active still somehow. 2) Diagnostic lights 1 and 3 are on when the ram is installed in 1 or 3. 3) Removing the CPU shows about 3 or more pins bent (I didn't count I'm going off by memory sorry) I'm not sure if they have anything to do with the slots. 4) When I found it had a fair amount of dust inside the computer and I did clean with isopropyl alcohol (50%) with a Q-tip and did blow on slots 1 and 3 if any dust was inside but as far I'm aware it did nothing and perhaps made it worse. 5) With the bent pins I mentioned the PC works just fine nothing wrong as far I'm aware of it's just DIMM 1 & 3 that are not working. Please I need some help even if I can't do anything to fix it I'll consider a new motherboard because I'm planning on using it for games nothing too heavy something like archived flash games or games that can be heavy but can be adjusted to be lighter like Minecraft (not java because that takes to much resources) or any older titles.
Optiplex 790 DIMM 1 & 3 not working