HTT should never be above 1Ghz, in my personal experience.
A succesfull overclocking on an A64 includes lowering the LDT multiplier to fit the HTT speed of 1Ghz max. When going above this speed, very eratical behaivior can happen. For example, my ATI crashed as soon as I loaded WinXP Pro, and had to use VPU recover if my HTT was above 1Ghz.
However, not all motherboards are created equal, and some will be able to do above 1.4Ghz HTT, but the general rule is "no more than 1Ghz". Else, everything is fine.
LDT X FSB = HTT