A couple of things that can cause RAM error - mismatched RAM.  Sometimes it may run, but at reduced performance.  This is if either stick works, and if that's the case, possible bad DIMM slots.  Try known working stick in each of the DIMMs. Yes, migrate over to a DT if that's what you want to do.  If you move HDD/SSD, Win7 or 10 will detect new hardware and will ask to be reactivated online.  You can use Win7 license key one time for Win10.  Much better performance with SSD if not already.
A little recent background: This system has been running fine until I tried to add a video card which has been maybe a bit too much since it didn't boot at first, then booted but screen blacked out after being on 30 min. (and of course I no longer have that in there) That was a few weeks ago but last night #3 and 4 LEDs was on. Upon searching online I tried pulling the newer of the two stick of RAM (which I've had a good while, longer than the video card) and it seems to reset the system temporarily, and I'm able to put both modules back. But after shutting down it happened again the next time it boots up. Whether it's the mobo or the PSU I have a few spare Optiplex 3010 DTs but I think this may be the only SFF. I'm thinking it may be the power supply but considering it's not the same form factor it may make more sense if I migrate my system over to one of the DT systems. This leads me to wonder if I can simply plug my drive in one of the other systems and maybe change the Windows license key, though I'm thinking it may involve upgrading from Win 7 to 10 first. Even if it will consistently work with one stick of RAM or 4 instead of 8gb I'm guessing this system is on the way out. Thanks for reading.
Optiplex 3010 SFF LED 3&4, works w/1 stick of ram - Migrate to DT?