its old

what does better , mean,  ask 100 folks get 100 answers and even all correct.

name one thing you do that needs more processing power, (gaming?) tell what is slow or why ask.

in most cases no gain, for most folks, but... many times its some gamer with old/slow/HOT video card.

runs core2 quad, best.

chipset  Q45. limited. as all are.

first hit at dell,  this.  q9650 stated. clearly.

no goal stated, why this? gaming or what, tell what you do that is slow, name one thing, as many times all this effort gives you no better, performance doing "That thing"

https://www.dell.com/community/Optiplex-Desktops/780-CPU-upgrade/td-p/6103097

Is your "mini" the MT, DT, SFF, or USFF?

The max CPU is Intel Core 2 Quad Q9650 except in the USFF.  The Q9650 has 64% better performance than the E8400 according to game-debate.

Modded Xeon's that pop up on eBay aren't recommended.  The QX9650 - NOT - but it may also pop up.

SSD's also breathe new life into older machines.

Optiplex 780 Tech Guide

Hi there, 

The CPU supported and tested for Mini tower chassis are listed below. We don't recommend any other CPU upgrade as it can cause issues to the system hardware, also it can have compatibility issues.

https://www.amazon.com/Intel-Q9650-Processor-Socket-LGA775/dp/B001D86S3G/

Welcome to the Dell Community @SetLucas 

Intel Core 2 Duo E8400 specifications and upgrades.

http://www.cpu-upgrade.com/CPUs/Intel/Core_2_Duo/E8400.html

The E8400 is based on Wolfdale core, and it utilizes Socket 775.

Intel Core 2 Quad Q9650 specifications and upgrades.

http://www.cpu-upgrade.com/CPUs/Intel/Core_2_Quad/Q9650.html

The Q9650 is based on Yorkfield core, and it requires Socket 775.

The TDP for the Q9650 is much higher!!! "E8400 65w -Q9650 95w"

The "Steppings" are workable!!!

Before doing any Memory/CPU upgrade make sure your BIOS is up to date.

Drivers & downloads​ for OptiPlex 780

https://www.dell.com/support/home/au/en/aubsd1/product-support/product/optiplex-780/drivers

Best of luck,

U2

The Q9650 is what I use in my 755 with the same big high performance heatsink that it came with.

Check out my other thread if want:  https://www.dell.com/community/Optiplex-Desktops/755-MT-Win7-to-Win10/td-p/7281716

OMG thank youuuu!!!! I'm so happy you exist

hello i have the same problem as i want to upgrade my ram and thinking about to upgrade the Intel duo core to something more fast cpu