I've just placed an order at my local whitebox shop for my long-awaited new desktop, a quad-core beast.
Pending some major change to the way that we create software, quad core seems to me like the first major sweet spot for multi-core.
You have enough power to overcome the overheads of doing parallism and still get a major speed boost.
On a typical Win32/desktop/whatever PC, you also have enough spare CPU for doing two CPU-hogging tasks, spare room for OS maintenance stuff, and another for random mp3 players and so on.
The next major sweet spot after 4-way, judging by the Wide Finder competition, is 32-way parallelism. Both the most competitive programs in that competition are faster 32-way than they are 64-way.
I assume that certain overheads in dealing with the quantity of CPUs starts to bite harder at that point.