In traditional overclocking, that's true. However, there are FSB:Memory ratios that you can adjust to allow you to drop the memory's speed and continue cranking up the FSB.
With the Athlon64, there is no FSB. Since there's no memory controller with A64 chipsets, a large part of the reason for the northbridge is eliminated...along with the FSB. Since the memory controller has to run at a divider (since we don't have 2-3GHz DDR/DDR2 modules around). You can run the RAM slower than it's rated speed and not suffer drastic performance hits like you would with traditional designs, particularly with the AthlonXP.