Hi. I using a Dell Optiplex 790 SFF and my friend just give me 2x2GB RAM (different type with the stock one)
When i install it and booting the pc up, the screen show:
Alert! Rear Fan Failure
The amount of system memory has changed
Is it something wrong with my PC or i just ignore it and press F1 to continue boot?
The system fan is attached to the connector not on the cpu.
SFF vs USFF is different
My guess is that you accidentally disconnected the cable connecting the rear fan(s) to the motherboard whilst you were upgrading the RAM. Or perhaps you had to do the disconnect to get to the RAM slots but forgot to reconnect when reassembling. If the connection _looks_ good, take it apart again, squirt some contact cleaner on the ends of the cable & reconnect.
I used to work on computer hardware for a living, I know how easy it can be to do this sort of thing & also how effective the contact cleaner trick can be.