I installed a Core2Duo in my GX620 and it refused to boot or even POST. I installed a Pentium Dual Core in my GX620 and it refused to boot or even POST. Neither of these processors will work. They are from a different series of processors. I know it is not on the approved processors list from Dell. Yes it will take a Pentium D which is dual-core, but it is a different flavor of dual core. Yes the Intel 945 Express chipset on the motherboard should be able to run these CPUs. But a C2D is incompatible with the GX620. The BIOS will not allow it.
Yes I tried it. No it does not work. Yes everyone is right that you should probably spend the money on a motherboard and processor bundle and upgrade instead of changing your processor . . . but I had these in house so I did it.