I suggest you stick to processor models that originally shipped inside each of those models respectively.
You not only have to account for processor socket, but also included CPU micro-code in BIOS (ie, BIOS support for each CPU model). Possibly even specs for any installed RAM DIMMs (if trying to reuse them).
The problem with CORE I3 I5 I7 is that they swapped and swapped and swapped from Socket 1366 to 1156 to 1155 to 1151 to 1150 etc
The 790 and 990 are LGA1155
Whether or not you can use a specific part DEPENDS on what SIZE and what socket the system has and the chipset.
aka MT DT SFF USFF
The USFF can ONLY USE 65w PROCESSORS.
Intel® Core™ i7 2600 / 3.40GHz, 8M, VT 95W
Intel® Core™ i5 2500 / 3.30GHz, 6M, 95W
Intel® Core™ i5 2400 / 3.10GHz, 6M, VT 95W
Intel® Core™ i7 2600S / 2.80GHz, 8M, VT 65W
Intel® Core™ i5 2500S / 2.70GHz, 6M, 65W
Intel® Core™ i5 2400S / 2.50GHz, 6M, 65W
Intel® Core™ i3 2120 / 3.30GHz, 3M 65W
Intel® Core™ i3 2100 / 3.10GHz, 3M, 65W
the Dell Optiplex 790 (MT=Mini-Tower) motherboard spec reads its
Intel quad Core i-7 2600 / 3.40GHz, 8M, VT-x, VT-d, TXT (vPro), 95W X-GSP
Page 12 https://clascsg.uconn.edu/download/specs/O790.pdf
There is also an i-7 2600S which seems to be for motherboards of the USFF(smallest)
by the diagram I would conclude this 2600S can not be used by any of the other Optiplex 790 types.
thoe if anybody could answer me if optiplex 790 are able to use i7-2600K?
what is difference between i-7 2600 and i-7 2600K?