Dell XPS 420 CPU upgrade

P6600 was a typo, I'm glad you understood what I meant though.

 To update with a little more info, I found that the Q9540 is compatible with the computer's X38 chipset, but I'm not knowledgeable enough to know if that means that it will work with this specific computer.

https://www.intel.com/content/www/us/en/support/articles/000007669/server-products.html

There is so little information out there on this computer. I even tried to look up the bios info to see if it had a compatibility list to no luck.

Edit: I know it's old. I want to run windows XP off of it to play games from 2002 to 2009 without many compatibility issues like I would if I tried to play them off modern hardware.

I also want to upgrade the CPU for the more modern SSE 4.1 instructions.

No. IIRC, it has something to do with ram-speed, chipset-speed, or the microcode for other processors simply isn't in the BIOS. 

Basically, you run the dual-cores e6600 and e6700 or
the quad-core (about twice as fast actually) q6600 and q6700.

Never heard of a Core2Quad P6600

https://en.wikipedia.org/wiki/List_of_Intel_Core_2_processors

Finally, swapping one Core2 quad for another quad is of little real-world benefit. Also, that machine is very old.

Thank you!