Do not confuse heat issues wiht Athlon XP chips and the new Athlon 64 based chips, they're completely different.
Althon XP has heat issues because the core itself was very small and was used to dissipate the heat directly to the heat sink. This meant a small surface area was used to transfer the heat, which is less efficient. Combine that with the upwards of 90W dissipation of some models (Athlon1400, .15u XP 2000+, and generaly every top clock CPU before a process downsize) and you had some users inadvertantly burning up their CPU's. Also, thermal protection was in it's infancy so a loose heat sink of failed fan meant certain death.
Athlon 64's have the large heatspreader over the core, like the P4. This leads to a much better contact point for dissipation to the heat sink. Also, the dissipation required is anywher in the mid 50's for the lower clock models to upwards of about 80W for the speedier chips. Compare that to the space heater like dissipation requirements of the Prescot core Pentium 4 at 100W or more, and the Athlon 64 starts to look like an ice cube. And now that the thermal shutdowns are implemented, you can have a heat sink fall clean off a Athlon 64 and the PC will shut down before any damage is done to the CPU.