They will under volt easy peasy! could also just leave speed step enabled, which will under volt and under clock when the cpu is not under load saving power and decreasing heat.
Passive cooling would probably be possible with these chips.
Another alternative if you want a silly power efficient Core 2 system could maybe be a MoDT solution. The chipsets used on these special motherboards roughly draw around 10-15 watts less power than a modern desktop chipset but still have all the functionally: many sata ports, pci-e slots, gigabit lan etc...!
Pop in a mobile core 2 solo processor and you’re looking at a system that draws less than 40 watts total on full load!!
It’s also worth noting a mobile core 2 duo will draw equal or less power than a desktop core 2 solo.
For example, a 1.66GHz T5500 mobile core 2 duo has a TDP of just 34w matching that of a 1.6GHz single core 420 celeron. Also as the T5500 has twice as many cores and four times the L2 Cache it’s not hard to work out which is the faster processor!
Basically the only drawback of this MoDT hardware is its expensive
I get around this by picking up second hand gear as it tends to go by unnoticed. The rest of the components are just from modern laptops which is readily available and silly cheap.
It might even be cool enough to go passive.