Well, that comparison is unfair. A Pentium D es much slower than a C2D even if they have comparable clock speed. But anyway, I think that the emulation of the current generation machines won't be possible at least for a long, looooooong time, because their power is nearly on par with current PCs and because they are very complex.
Anyway, I'm pretty sure there are still some slower systems that will eventually be emulated (some year..) such as the GC (the last version of Dolphin got quiet far), the DS (actually, the DS emulators are even in playable stages for a lot of games), the PSP or even the Wii. But even if this never happens, there will allways be ports of the old emulators for newer systems