The Pentium D is 2 x Pentium 4 Netburst architecture chips on the same die. Core2 is a entirely new architecture, The 2.2Ghz Core 2 is faster than the 3Ghz Pentium D. In fact a 3Ghz Pentium D is roughly the same as a 1.4Ghz Core2 chip even though both chips are dual core.
Both are very obsolete at this point.