Thanks for the quick response, and the reference to the similar question asked by Ater_Swe.
My current setup is:
Intel® Xeon® Processor 5140 (4M Cache, 2.33 GHz, 1333 MHz FSB)
HP 4 GB (2 x 2 GB) PC2-5300F DDR2-667 ECC Fully Buffered DIMM
Is this the faster CPU you recommend:
https://www.amazon.com/Intel-1333MHz-LGA771-Quad-Core-Processor/dp/B000K1KTS2
(Intel Xeon E5345 2.33 GHz 8M L2 Cache 1333MHz FSB LGA771 Active Quad-Core Processor)
If so, the new ones are expensive at $500, but Amazon lists used or refurbished ones for $25-$30.
Do you think swapping out the current CPUs with those will make a noticable performance difference?
Also, so I understand that those CPU will fit in the current motherboard and heatsink, as explained in the HP xw8400 Service and Technical Reference Guide -- page 93 ("Replacing the processor").
http://h50146.www5.hp.com/lib/doc/manual/workstation/xw8400/364898-001.pdf
------------------------------------
Replacing the processor
1. Disconnect power from the system ( Pre-disassembly procedures on page 55). Remove the access panel (Access panel on page 60), the CPU heatsink (Removing the CPU heatsink on page 88), and the processor (Removing the processor on page 91).
2. Raise the processor socket handle fully (the full swing angle of the lever is approximately 105 degrees).
3. Align the notches in the processor base with the tabs on the socket. Seat the processor into the socket. Ensure that the underside of the processor is level with the top of the processor socket. Close the processor cover plate. Lightly press down on the processor cover plate while closing the socket lever.
-----------------------------------
Do you also recommend upgrading the memory? If so, any specific model/specs?
I saw your comment to Ater_Swe:
best to populate all memory slots with identical RAM sticks, HP holographic label ones, and ideally from the same vendor such as Elpida or Nanya (my favorites for that era, but Samsung is close behind as is Hynix. Not Micron for the 4 GB sticks.) Look for HP part number 398708-061 for 4 GB sticks. For 2GB HP sticks look for the same brands but Micron 2 GB are OK. Look under 398707-051 (or -551).
Thanks again for your help!