What is or isnt supported DEPENDS on bios version AND motherboard version AND Size aka MT DT SFF USFF.

Generally speaking USFF does NOT support Quads due to insufficient power supply.  Because these are sooo old now the processors for LGA 775 are very inexpensive now on the correct sites.

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

https://www.dell.com/downloads/global/products/optix/en/optiplex-380-tech-guide.pdf

Validated units include the following for MT DT SFF

Intel® Core™ 2 Quad Processors
Intel® Core™ 2 Quad Q9650/3.00GHz, 12M, 1333FSB
Intel® Core™ 2 Quad Q9550/2.83GHz, 12M, 1333FSB
Intel® Core™ 2 Quad Q9505/2.83GHz, 6M, 1333FSB
Intel® Core™ 2 Quad Q9400/2.66GHz, 6M, 1333FSB
Intel® Core™ 2 Quad Q8400/2.66GHz, 4M, 1333FSB

BIOS should be updated before changing CPU's.  Use the "Drivers & Downloads" link below.

If the system is 32-bit and not 64-bit, a quad CPU won't be fully utilized, only 2 cores will be.

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@bradthetechnut wrote:

BIOS should be updated before changing CPU's.  Use the "Drivers & Downloads" link below.

If the system chipset is 32-bit and not 64-bit, a quad CPU won't be fully utilized, only 2 cores will be.

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There is absolutely no such thing as a 32 bit chipset.

Almost all the CPUS that came out after 2005/2006 are EMT 64 aka 64 bit cpus.

There is also no such thing as only 2 cores on a quad being used vs 4 because all the Quads are 64 bit processors.  Not sure what your basis is for calling a chipset 32 bit. That's nonsense.

Core DUO a long time ago was 32 bit only but again this has nothing to do with chipset.

The first processor to implement Intel 64 was the multi-socket processor Xeon code-named Nocona in June 2004. Initial Prescott chips (February 2004) did not enable this feature. Intel subsequently began selling Intel 64-enabled Pentium 4s using the E0 revision of the Prescott core, being sold on the OEM market as the Pentium 4, model F. The E0 revision also adds eXecute Disable (XD) (Intel's name for the NX bit) to Intel 64, and has been included in then current Xeon code-named Irwindale. Intel's official launch of Intel 64 (under the name EM64T at that time) in mainstream desktop processors was the N0 stepping Prescott-2M.

NX is important because its required for windows 8 and 10.

LAHF SAHF and CMPXCG 16 are required for 64 bit versions of windows 8.1 and 10.

The first CPU that does all of these is the Pressler Pentium D 900 series from 915 to 960.  These cpus work in models as old as the OPTIPLEX GX620.

 

@speedstep,

It was in this forum that I learned that 32-bit Windows doesn't fully utilize a quad processor, nor RAM beyond 4GB.  Therefore, it wouldn't pay to put a Q9650 processor in my 755 MT (running 32-bit Win7) and 8 GB of RAM.  If this needs correcting, no need to give a hard time.  You're a Rockstar and therefore a pro. 

What I don't know is why your posts are in large text.  Like photos, it only shows when signed out.  Signed in, text appears normal - same universal size as everybody else's.

" 32-bit Windows doesn't fully utilize a quad processor, nor RAM beyond 4GB.  "

The 2nd part is correct. 

" doesn't fully utilize a quad processor" what does that mean?

There is no such thing as 32 bit only using 2 cores.

Thats nonsense.

If I remember right - So if somebody has 32-bit Windows and want 64-bit, besides changing to a 64-bit CPU, they'd have to reload Windows?

What is the 2nd item (not the CPU) on the motherboard that always has a smaller heatsink, isn't that the chipset?

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@bradthetechnut wrote:

If I remember right - So if somebody has 32-bit Windows and want 64-bit, besides changing to a 64-bit CPU, they'd have to reload Windows?

What is the 2nd item (not the CPU) on the motherboard that always has a smaller heatsink, isn't that the chipset?

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that's right, bradthetechnut.

a "clean" install of 64bit Windows is required when overwriting the 32bit Windows (you have to boot directly with the installation media whether a CD or USB flash drive as you cannot perform the upgrade from within Windows when attempting to upgrade from 32bit to 64bit).  I did this myself on an old Dell Inspiron 640m/e1405 laptop computer, which used to come with an Intel Core Duo (Yonah) CPU that was 32bit only and I replaced the CPU with an Intel Core 2 Duo (Merom)

Thanks for replying @erpster05.  So I'm asking this because of the way I worded my 1st question above - If I switch from an Intel Core 2 Duo E8400 to Intel Core 2 Quad Q9650, I can't just leave Windows the way it is, I have to reload no matter what?  Thanks for not minding that I verify.

Could I get away with backing up the Windows I currently have or is Windows Restore an option?

There are NO UPGRADES for ANY version of windows from 32 to 64 or 64 to 32..

Either scenario requires a complete clean install of windows and all applications etc.