Compatible processors listed here....

http://www.dell.com/downloads/global/products/optix/en/spec_optix_gx520-gx620_en.pdf

Both are socket 775 LGA, so at least the pins (sic) match up...

A problem you may have is the operating system which was installed over the Pentium D/Pentium 4...whatever you have in your 'plex... The drivers (registry) may be handled differently between the processors.

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