What's wrong with a
processor running at 24 watts? The Pentium-M "Centrino" at 1.7ghz is running a robust 26-30w when solidly active, then scaling down to 10w when at an idle of 600mhz. This is on a 400mhz FSB (like the 750vx), and typically using PC2700 RAM (which Apple already does).
So, let's extrapolate a moment. A single G5 970fx at 1.8ghz is roughly 22-30w. Instead of having a lower-clock single chip, you could be pumping two higher clocked chips that are (clock-for-clock) competitively specced with the G5 in a lower heat and power profile. A single 750vx runs at 10-15w when going full bore, has power management that scales it down, possesses over twice the RAM bus of the G4, outperforms its predecessor at roughly 35-40% per clock, and is also a full 25% faster than the current G4s to being with.
In short, you get SMP benefits, a high-clock, low-power chip at 2.0ghz and possibly higher (the 750vx is reported to scale at lower power than the G5), and a far better FSB and memory bus than the existing laptops. On top of that, you neither kill the heat nor the battery profile, though you do increase both somewhat, perhaps equal to putting a single G5 under the hood.
I know what I would rather have, and it doesn't end in the number five.