Optiplex 745 motherboard in a GX620 case?

hum i see,

the front panel (power switch, usb and audio jack) cable are the same, fan cable are the same, and all motherboard connectors are in the same spot as gx620, so maybeeeeeeeee can work, maybeeeeee, the 780 motherboard only cost 15euros, worth the risk

I disagree entirely.

These models are not the same and  do not have removable back plate.  This means you would have to hack holes in the back to get it to fit at a minimum.  The power supply is NOT the same rating between the different models on all rails.  The 745 has 2 audio outputs not 3.  The boards are not interchangeable between models OR sizes.The 745 uses SATA power and SATA optical drive the 520/620 uses ATAPI Optical drive and MOLEX Power connectors.  So there are several physical incompatibilities as well as electrical issues.

(power switch, usb and audio jack IS NOT THE SAME)

Different part number and pinout for GX620 745 755 760 780 980

cable are the same, fan cable are the same (NO they are NOT)

Again I disagree.Different part number for GX620 745 755 760 780 980

 DCTR, DCNE, DCSM, and DCCY

As for the 780 and 980

YMMV

Here are the pin mappings for a Dell Optiplex 980 front panel connector:

Power On Switch: Pins 1 and 14

The Power LEDS and Faults 1-4 LEDS all use a common Positive Terminal, so:

Power Blue: Pin 2 (negative), Pin 3 (positive)
Power Orange: Pin 12 (negative), Pin 3 (positive)
Fault 3: Pin 4 (negative), Pin 3 (positive)
Fault 1: Pin 5 (negative), Pin 3 (positive)
Fault 2: Pin 10 (negative), Pin 3 (positive)
Fault 4: Pin 11 (negative), Pin 3 (positive)

The Status lights for WiFi, Wired Networking, and the Hard Drive all use the same Positive Terminal, so:

Wired Networking: Pin 6 (negative), Pin 8 (positive)
Hard Drive: Pin 7 (negative), Pin 8 (positive)
WiFi: Pin 9 (negative), Pin 8 (positive)

Pins 13 and 14 seem to be connected - no resistance between the two.