OP:
Basically, Core 2 Duo's are faster, per clock, than A64 X2's, Core Duo's, and Pentium D's. Pentium D's are old and inefficient now, an option that I would personally not even consider anymore. For desktop applications, I would do what I could to get a C2D as they also overclock like mad; most can reach upwards or past the 3GHz barrier, even the ones that come stock at 1.8GHz, without too much of an elaborate cooling setup. Granted there are quite a few A64 X2's that can reach that high, the C2D's at that speed would trounce any Athlon. The Core Duo's were notebook processors and hence would be a somewhat abstract choice for a desktop environment. Core 2 Duo's were sort of built off of Core Duo (hence the 2).