any recommendations on what I should use (what CPU?)No wrong socket. That motherboard only supports 2nd gen intel processors, the i5 8400 is 8th gen much newer. To use an i5 8400 you would at least need a new motherboard and DDR4 RAM.
I am using a dell optiplex 990 with a 06D7TR MotherboardWith the motherboard you currently have? What are you using now?
Using an I7 2600What CPU are you currently using? Already know your motherboard.