I've seen many benchmarks rate the top level stock Intel CoreDuo out perform their top level stock Quad Core, I've also seen the quad core cause many bugs. So more than any reason not to buy a quad core PC, is the technology just isn't there yet.
Multi-threading applications are the future (and more, perhaps parallel processing and streaming as well), dual and quad cores are amazing for multi-tasking, but as always, Intel's latest crack at a new commercial technology falls way behind the quality standards that we'd expect of other CPU technologies. If you're a big multi-tasker and want something with a bit more kick dual core (because I sure do), wait off for AMDs Barcelona core, load of info on it here:
http://anandtech.com/cpuchipsets/showdoc.aspx?i=2939It should be the same as this core architecture compared to AMD64s as Core2s were to Pentiums. I'm quite excited to see how early benchmarks fare.