Yeah MHz for MHz and volt for volt the Palominos run cooler, mostly because they have a lesser watt per sq mm ratio than the TBird. (meaning the Palomino's have a core with larger surface area and don't put out any extra heat).
This is why TBred1 is so much hotter than Palomino - it actually puts less watts out but the watts/sq mm ratio is higher because the core is so small. Dissipating heat is easier over a larger contact surface than a smaller one.
So, in theory, an XP 1500+ is the coolest running Athlon at stock speed/voltage. Early Durons ran very cool as well. I built several Duron 700 systems for friends with Chrome Orbs and those systems idled at 1 or 2 C above case temp. My Duron 700 with the KT133A register 52 hack idled at even temp with case temperature.
sb