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

"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
*************************************************************

q8700 ?  which cpu is that

"Q8700" looks like a typo.  May be somebody meant "Q6700"?

uh, no.  why would i want to experiment with an lga 1156 socket into a lga 775 socket

besides, there isn't a performance increase with this processor.  use the q6700

The Xeon x3230 is socket LGA 775 and has the same CPUID as the Q6700, 6FB.  NOTE:  This does not mean I can throw my support in putting a Xeon in an Optiplex.

What Trevor did not say is if he used an X3230 in a 745 himself.  Therefore, it's not said what needs to be adapted.  Some of it may look good on specs, but Xeon's typically don't work in Optiplex's as they're not built for it.  (This does not mean never, as some claimed to have made it work.)  Is an adapter needed?  Or does an extra notch need to be cut into the Xeon so it fits?  No link was provided to the site Trevor visited.

The wrong CPU, or trying to insert a CPU the wrong way can easily bend pins, causing further problems.

Many Xeon's don't come with integrated graphics, therefore requiring a separate GPU card.  No integrated graphics is mentioned for the X3230.  https://ark.intel.com/content/www/us/en/ark/products/30797/intel-xeon-processor-x3230-8m-cache-2-66-ghz-1066-mhz-fsb.html 

The xeon x3230 have it placed on my computer