780 DT & Q9500

http://i.dell.com/sites/doccontent/business/solutions/engineering-docs/en/Documents/optiplex-780-tech-guide.pdf

Specific Quads are supported  Q9650 is the top end.

Since the 9650 Yorkfield is supported I see no reason why it would not work.

@U2CAMEB4ME

  I find your advice to be false

https://ark.intel.com/content/www/us/en/ark/products/35428/intel-core-2-quad-processor-q9650-12m-cache-3-00-ghz-1333-mhz-fsb.html

https://ark.intel.com/content/www/us/en/ark/products/37159/intel-core-2-quad-processor-q9500-6m-cache-2-83-ghz-1333-mhz-fsb.html

Intel® Core™ 2 Quad Q9650/3.00GHz, 12M, 1333FSB 
Intel® Core™ 2 Quad Q9550/2.83GHz, 12M, 1333FSB 
Intel® Core™ 2 Quad Q9400/2.66GHz, 6M, 1333FSB 
Intel® Core™ 2 Quad Q8400/2.66GHz, 4M, 1333FSB 
Intel® Core™ 2 Quad Q8300/2.50GHz, 4M, 1333FSB 

Thank you for find that @speedstep 

I was assuming that the OP had already tried.

I did not look into it much further than that.

I guess they would just need to make sure they have the correct heatsink???

Is there any BIOS update that would be needed???

Regards,

U2

NOTE: As should be I will ask Admin to remove my post as solution!!! 

EDIT: Thank you for "EDITING" your post to reflect that!!!

I'm using a Q9500 in an MT-780 with A15 bios now as I type this. It's used as my primary office  unit  daily.  Have run  Heaven Benchmark many times and often do pdf editing with several windows open simultaneously.  I did the   swap  9 months ago running  Win7/64.   No problems but I expected more, maybe I expected too much? 

Over 1 year ago I replaced an E8400 in my  MT-755  used as  HTPC for gaming/Netflix.  It has  2 HDD's dual boot Win10-7.  The 9650  made for a  more noticeable  difference than the Q9500. Both computers  had E8400 and  8gb ram although 780 is ddr3 while 755 is ddr2.

Both computers had aftermarket  HD PSU's and  GPU's ( 780: Quadro 600 and 755: 750ti).

Hope this helps

Hi @U2CAMEB4ME,

In case you didn't run across the answers you were looking for by now...

It's always recommended to update to the latest BIOS first before upgrading CPU's.  Latest BIOS versions vary by model.  While A22 is the latest for mine, A17 might be the latest for another.  Dell's Drivers & Downloads site automatically scans PC and shows which BIOS to download.

Heatsinks are already made to handle the top end CPU within specs. - in this case the Q9650.

Hi @hjkp,

You can definitely get more speed out of your 780 & 755 by having the OS on SSD's.  SSD's really do breathe new life into older machines.  As for expecting more - you were right.

When I maxed the RAM and added an SSD to my 755, even my lagging and disappointing USB's speeded up, including USB 3.0 which I added to it.  I also switched from Win7 to Win10 at the time.

In case it needs stating here - One can't always just jump to the latest version of BIOS.  Mine came with A09, and I had to download 1 or 2 BIOS before getting to A22.

Thanks @bradthetechnut 

Best regards,

U2

You're welcome @U2CAMEB4ME.

I got my first SSD in December and  it improved  my HTPC  experience with speed and being quieter, too.

The Dell MT's  last a long time.  I replace PSU's occasionally yet  the boards keep going.  The  HTPC unit is loaded so I added a front  cooling fan but I see no reason to upgrade from 775 just yet.  The only major advantage with  newer CPUs is efficiency and that's not an issue at home. I'll wait for a better 5.1 Surround  platform before I orphan my 775's. Ned

These models work with windows 10 but they have Spectre and Meltdown issues that will never be patched.

https://www.grc.com/inspectre.htm

Intel decided that it will not be issuing Spectre patches to a number of older Intel processor families, leaving many customers vulnerable to the security exploit.

https://ark.intel.com/content/www/us/en/ark/products/series/79666/legacy-intel-core-processors.html

The processors which Intel won’t be patching include four lines from 2007, Penryn, Yorkfield, and Wolfdale, along with Bloomfield (2009), Clarksfield (2009), Jasper Forest (2010) and the Intel Atom SoFIA processors from 2015.

The 9500 is a yorkfield unit.

https://ark.intel.com/content/www/us/en/ark/products/37159/intel-core-2-quad-processor-q9500-6m-cache-2-83-ghz-1333-mhz-fsb.html