> One thing that is not clear to me is how much more power efficient (and the more > important derivative of this, quiet) the i7 is.
The i7 drops to 1.6 GHz and starts turning off cores basically the instant you quit MAME, so it's by far the most livable overclocked processor I've ever used. It's a significant turnaround from the days of the Pentium 4, to put it mildly.