What you say is true about lower bandwidth. You will have lower bandwidth. However, you do not suffer the massive latency penalty that the AthlonXP did if memory and FSB were not at the same speed. That was the main reason for concern previously, but is not a real issue with the introduction of the integrated memory controller in the CPU.
You can usually lower any loss in performance from having lower bandwidth by tightening the timings as well as increasing the overall CPU clock speed.
Overall clock speed is more important than having an extreme amount of bandwidth (assuming you're not lowering the memory speed down to nothing, of course).
The Athlon64 architecture, at this point, isn't really starved for memory bandwidth.