Historically, the Athlon 64 hasn’t really been memory bandwidth bound, since the move to Socket-939, which gave it a full 128-bit wide memory bus, and more bandwidth than these CPUs could use.
With the move to dual core however, the effective memory bandwidth that each core gets is significantly reduced, as they both have to share the same 128-bit wide memory interface normally dedicated to a single processor. So in theory, the new dual core X2 line of processors could be a good candidate for these new memory dividers.
The other situation where higher clocked memory is important is with higher clock speed CPUs. The faster that your CPU clock gets, the quicker it can process data and thus, the faster that it needs information and the more memory bandwidth that it needs.
The lower clocked CPUs are less likely to see any real performance difference, with DDR400 being more than sufficient for their needs.