Question : Pentium D 820/840 compatibility

I have a Dell 9100 with a Pentium D 820 CPU.  A friend has a Pentium D 840 CPU, can I just swap, or are they too different?

Thanks.

Answer : Pentium D 820/840 compatibility

yes you can change them.. here is the result of comparison between them
http://ark.intel.com/Compare.aspx?ids=27512,27514,