|The trick to overclocking an A64 system to to find the right combination of the CPU multiplier, CPU reference clock, memory & HT settings, plus the PCIe must be locked at 100MHz.|
The HT bus speed is the product of the CPU reference clock x the HT multiplier. The default setting is 5 x 200MHz = 1000MHz. If you want to increase the reference clock to say, 250MHz, you'll have to drop the HT multi to 4x, so that the HT bus stays at 1000MHz (4 x 250MHz). You'll also have to lower the memory setting & will probably have to lower the CPU multi as well.