With the exception of the AMD FX series processors, the FSB multiplier on AMD 64 processors cannot be raised above the the value that corresponds to the CPU's rated speed. For example, for an Althon 64 Winchester 3500+, which has a rated speed of 2.2 GHz, the multiplier cannot be set to a value greater than 11 (since 11x200MHz = 2.2 GHz). For FX processors, the multiplier is NOT limited (thus, for the FX-55, it may be raised above its "normal" value of 13).
Note that for ALL AMD 64 processors, there's no lock in the other direction, and the multiplier may be lowered below the normal value.