If you don't intend to overclock there's really no point in buying faster memory unless you want to lower the multi and raise the FSB, and even then AMD64s have enough memory bandwidth anyways with LDT.
Despite popular belief, it is the CPU which dictates memory speed, not the motherboard. A motherboard supporting DDR will support any DDR speed, however the CPU FSB has to be able to support than speed.
On Athlon 64s, the memory controller runs the RAM at 200MHz, therefore you want PC3200 as you mentioned you were looking at.