Re:  1) Is that i7 compatible with that motherboard? I know it fits socket but I read some cpu's might still not be compatible. i7-2600 is a standard Intel 2nd gen cpu and is supported by optiplex 990 regardless of motherboard DP/N. 2) Do I need to update BIOS and where can I find that update? no, you do not need to update bios.  given age of pc better not taking risk of flashing bios unless there is strong good reason. 3) If question 1 and 2 are ok to go. Do I install new cpu first then new Windows 10 or vice versa? you should first boot pc with current i3 and make sure it posts well. This will confirm all other hardware are in working condition. Then you can swap i3 for i7.  Power on to verify pc still posts well. Only then you can install Windows 10. This is analogous to building a house. First making sure foundation is solid before architecting upper high level structures. 4) Anything else you could think of I should know? you do not need more than 16Gb for gaming. 32 gb ram is more than enough.  A sata ssd will help booting faster.  Optiplex 990 top cpu is i7-2600. No need to consider -k cpu because Dell bios doesn’t let you overclock.  When you consider gpu upgrade for gaming, also consider upgrading stock psu.  
Hello, I just bought a optiplex 990 with i3-2120 LGA 1155 cpu. Motherboard has two sets of numbers. 1st one is: E93839 2nd one is: KA0121 Don't know which is motherboard is ID. Took a chance and bought a i7-2600 LGA 1155 cpu on offer up. I will be installing Windows 10 Home bought from ebay with new product key. My question to you are: 1) Is that i7 compatible with that motherboard? I know it fits socket but I read some cpu's might still not be compatible. 2) Do I need to update BIOS and where can I find that update? 3) If question 1 and 2 are ok to go. Do I install new cpu first then new Windows 10 or vice versa? 4) Anything else you could think of I should know? Thanks
CPU upgrade