As much as I love AMD, I would recommend against the Athlon64 chipsets, unless you *must* have a 64 bit chip.Why? They are much faster at running even 32-bit code than Athlons. They dissipate less power. They have safety features built in to prevent overheating, and power throttling built in to prevent less wasted energy when idle.
Perhaps the only reason not to move to the AMD 64 platform is the entry price, currently. The early adopters will take care of knocking that down for the rest of us.