MD,DT< oR SFF case?

why would you ,please tell what you do that the CPU change would cure,

gaming? name one thing slow please.

h61,  chip set it is runs

 i5 Quad Core; Intel® Core™ i3 Dual Core; Intel® Pentium® Dual Core; Intel® Celeron® Dual Core.

dells does not test all CPUs made on all 390s , only what the certified long ago.

Intel® Core™ i5 2400 / 3.10GHz, 6M, VT-x, 95W

that is it , the fastest one.

Sandy Bridge" (quad-core, 32 nm)

2550 is tops

but the i7-2700 is really the top top, right> and not supported by dell testing

but  95watts is max …. you could buy one and test it.

like most do , wanting too, what works for you , IDK but others here have posted on  sandy,  google that.

https://en.wikipedia.org/wiki/Sandy_Bridge

warning one Pentium is only 35watts and the heatsink can be for that and fail with the 95watt you posted.

https://i.dell.com/sites/csdocuments/Shared-Content_data-Sheets_Documents/en/Optiplex-390-Technical-Guidebook-101311.pdf

Intel Core i5-2400 Quad-Core Processor 3.1 GHz 6 MB Cache LGA 1155 BX80623I52400  is max  validated 2500k uses too much power

https://www.amazon.com/Intel-i5-2400-Processor-Certified-Refurbished/dp/B07HZ5CBXH/

https://www.amazon.com/Intel-Quad-Core-Processor-Certified-Refurbished/dp/B07HB5189S/