The Q9400 was never validated by Dell on the XE so isn't/wasn't supported, but having said that, it is compatible with the Q45 chipset which is used on the XE

It does support 8GB of RAM (1333 MHz DDR3 (Non-Ecc) )

If you are going to go ahead with the upgrade make sure the BIOS is at the latest version

So is it safe to assume the Q9650 will also work? Is it in the same socket and family as the Q9400, but higher clocked and faster than the Q9400.

Yes, it's the same socket type and is also compatible with the Q45 chipset. It should work but it's not something I can guarantee as, like the Q9400, it was never validated by Dell.

Here's the full list of CPUs that the chipset supports:

http://www.cpu-upgrade.com/mb-Intel_(chipsets)/Q45_Express.html

If you haven't purchased it yet get an Optiplex XE2 instead with  i7-4770s

https://www.amazon.com/Optiplex-Business-i7-4770s-Certified-Refurbished/dp/B07HKQ35LN

you must use optional 315w power supply for SFF XE

Pentium® Dual-Core E5300 is listed.  No mention of ANY Quad CPUs

315W 4FCWX D315ES-00 Power Supply

Intel Q45 Express Chipset w/ICH10DO

These are listed as compatible with the chipset.

Power Supply and Bios should be updated to current BEFORE trying to change cpu.

https://dl.dell.com/FOLDER01180418M/1/OXE-A05.EXE

https://dl.dell.com/FOLDER00540611M/1/OXE-A04.EXE

Ram must support 1600Mhz for quad cpus slower Slower ram 1333, 1066, 800mhz wont may not post. 95W cpus need 300W minimum power supply.

Intel® Core™2 Quad Processor Q9650

Intel® Core™2 Quad Processor Q9550S

Intel® Core™2 Quad Processor Q9550

Intel® Core™2 Quad Processor Q9450

Intel® Core™2 Quad Processor Q9400S

Intel® Core™2 Quad Processor Q9400

Intel® Core™2 Quad Processor Q9300

Intel® Core™2 Quad Processor Q6700

ntel® Core™2 Quad Processor Q6600

https://ark.intel.com/content/www/us/en/ark/products/36527/intel-82q45-graphics-and-memory-controlle...

https://i.dell.com/sites/csdocuments/Shared-Content_data-Sheets_Documents/en/us/optiplex-xe-tech-gui...

The E8600, Q9550S, and 95W Q9650 were not tested by Dell on this model.

Dell tested parts page list for this model =

4HF4Y Wolfdale E5300, 2.60GHz, 2MB, 65W, R0
38G73 Wolfdale E7400, 2.80GHz, 3MB, WFD, R0
H399J Wolfdale E8400, 3.00GHz, 6MB, 1333FSB, 65W, E0

Ram must be low density un buffered ddr3 1600 2gig x 4 for 8 gigs

https://www.amazon.com/Crucial-PC3-12800-Unbuffered-240-Pin-CT25664BA160B/dp/B006YG88QC/

Slower ram 1333, 1066, 800mhz wont post.

User benchmark has validated both Q9400 and Q9650 work in XE.

https://www.userbenchmark.com/System/Dell-OptiPlex-XE/13799

They Do not list the POWER SUPPLY or Case Size or bios version.

Going from 45w to 95W cpu REQUIRES at least 300W power supply.

280W is too small,

Optiplex XE Tech Guide

Optiplex XE2 Tech Guide

Optiplex XE3 Spec Sheet 

For anyone who might want to ask this question in 2024 or later, I can confirm that OXE works fine with Q9400S (lower wattage version of Q9400) with 8GB RAM, while the device gets way too hot with original Q9400. Noticeably smoother and snappy, even with lower wattage.