As far as the physical socket, you can probably change.
It is a PGA478
https://ark.intel.com/content/www/u...0-1m-cache-2-20-ghz-800-mhz-fsb-socket-p.html
Here are CPUs compatible with that slot:
cpu-benchmarks.com
Next question would be...does the BIOS allow for a different CPU? Maybe, maybe not.
Next question would be....Is the internal cooling situation viable for a different CPU? Completely unknown.
The T4400 TDP is 35W. A Core2 Extreme might be 130W. Major difference needed in the cooling situation.
Last question is....would you see a
real performance change? Possibly not.
AIOs are basically laptops in a...