This machine has an Intel Q965 chipset. So that means this machine won't take 1333mhz FSB CPU. The Intel Core 2 Quad Q9400 is 1333mhz FSB. Best CPU for this system would be: 

Intel Core 2 Quad Q6600

1066mhz FSB CPU does work though.

 

 

 

Dell Optiplex 745 Desktop DT Motherboard Intel Q965 (ICH8)

You cannot use Q8000 or Q9000 series only 6000 series and only 1066mhz fsb 65nm.

https://www.dell.com/downloads/global/products/optix/en/opti_745techspecs.pdf

 

PROCESSOR  1066MHz FSB is max for ANY cpu. 65nm is fine 45nm is not. 

Intel® Core™2 Quad Processor Q6700 is max with Max Bios.

https://www.youtube.com/watch?time_continue=3&v=qnmTJpCGzyE

Bios should be updated BEFORE changing CPU.

https://downloads.dell.com/FOLDER00224415M/1/HEIDEN_2.6.6.exe

 

Intel® Core™ 2 Duo  1066MHz FSB Socket T with  Dual Core technology XD, EM64T, 2MB and up to 4MB L2  cache, EIST and  VT (E6000 series)
Intel® Pentium® D 800MHz FSB Socket T with Dual Core technology XD, EM64T, 2x2MB L2 cache, EIST, and VT on 950/960 (900 series)
Intel® Pentium® D 800MHz FSB Socket T with Dual Core technology XD, EM64T, 2x1MB L2 cache (800 series)
Intel® Pentium®  4 800MHz FSB Socket T with HT, XD, EM64T, and 1MB L2 cache (500 series)
Intel® Celeron® D 533 MHz FSB Socket T with XD and EM64T (300 series) 

 

Thanks everyone for the replys i read everything and will start looking for best deals on what is able to be used

 

Despite what some have said, after FIRST installing the current BIOS, which last I checked was, I think, 6.6.6 or something like that, I was able to put a QX6700 in my grandmother's Optiplex 745. I can confirm it worked fine for over a year with no issues up until the point I bought her a new one. As my dad was given one identical to it by me at the same time and still has it in use, I'm actually planning on making that upgrade on it also, as he's giving it back to me so I have a PC both in my dorm and at home. Due to financial issues, depending on my work hours, it may be up until the end of September or beginning of October when I can afford to do so as, well, college is expensive.    Currently it only has a GT 730 2GB GPU in it, but it should suffice for now. That was just the best one I had lying around back then. A few paychecks after upgrading the CPU, I'll upgrade the GPU as well. The ram is current maxed out at 8GB (4×2GB) DDR2, though I do not recall the clock speeds. I want to say it was 667MHZ but I am not positive. Regardless, hopefully this helped some, and as soon as I can afford to do so, I'll proceed with the upgrade, take screen shots, including using CPU-Z, and set things up to where I can use my website to host them to this forum post.     Posted 8/25/2019 by Joshua Wigginton (BiggWigg92)

"QX6700" is nonsense and would not have worked.

Core 2 Extreme processors are 130W and 150W.

Precision T3400 units having these processors have 375W PFC base unit and 525W power supplies for the Core 2 Extreme not the stock 305w or less that the 745 comes with.

https://www.dell.com/downloads/global/products/precn/en/precn_t3400_specsheet.pdf

 

 

 

You're absolutely right, I did not intend for "QX6700" to have been what was submitted but I guess autocorrect got the better of me due to laziness in not verifying what I typed before submitting my response. What I actually got to work was the "Q6700" and it worked fine. That being said, I would recommend you go with either a "Q6700" or a "Q6600," depending on your budget, needs, and heatsink/cooling capacity.

Anyways, many apologies for my major blunder.

Here are some Intel Quad Core Processors that will work on Dell optiplex 745
Core 2 Quad
Q6600
Q8700

If you are using LGA771 to LGA775 mod, The dell optiplex 745 will support the below Intel Xeon Processors also
e5310
e5320

You will find it cheaper at NetGen Computers

Hello, the best processor for this model is intel xeon x3230. This I've found it in the page official of intel. Is real. This processor performs very well.

Thanks

I know this is an old thread, but I have a question. I have an Optiplex 745, and I meant to buy a Q6700, but I mistakenly bought a QX6700, and it works. The computer sees it as such. In SETTINGS --> ABOUT:  it shows an Intel(R) Core(TM)2 Quad CPU @ 2.66GHz 2.66GHz. And if I run the Intel(R) Processor Identification Utility, it sees it too (log at the bottom). My power supply appears to be the factory one, with 280W (pic below also) So, is it working, or is there something weird going on that I don't understand? I'm not a hardware person, but I am trying to learn and understand.

 

Intel® Processor Identification Utility
Version : 6.3.0404
TimeStamp : 2020/05/18 05:29:48
Operating System : Microsoft Windows NT 10.0.18363.0
Number of processors in system: 1
Current processor: 1
Active cores per processor: 4
Disabled cores per processor: 0
Processor Name: Intel(R) Core(TM)2 Extreme CPU QX6700 @ 2.66GHz
Type : 0
Family : 6
Model : F
Stepping : 7
Revision: 6A
Maximum CPUID Level: 10
L1 Instruction Cache: 4 x 32 KB
L1 Data Cache: 4 x 32 KB
L2 Cache: 2 x 4 MB
Packaging: LGA775
Enhanced Intel SpeedStep® Technology: Yes
MMX(TM): Yes
Intel® SSE: Yes
Intel® SSE2: Yes
Intel® SSE3: Yes
Intel® SSE4: No
Intel® AES-NI: No
Intel® AVX: No
Enhanced Halt State: Yes
Execute Disable Bit: Yes
Intel® Hyper-Threading Technology: No
Intel® 64 Architecture: Yes
Intel® virtualization technology: Yes
Intel® vt-x with extended page tables: No
System Graphics: Add-in Graphics
Expected Processor Frequency: 2.66 GHz
Reported Processor Frequency: 2.66 GHz
Expected System Bus Frequency: 1066 MHz
Reported System Bus Frequency: 1066 MHz
*************************************************************