Our overclocking tests were made with the CPU using its original cooler and standard voltage (1.425 V).
As we mentioned, the main difference between Athlon 64 and Athlon 64 FX is that FX has its clock multiplier unlocked, allowing you to make more aggressive overclocking. Actually Athlon 64 clock multiplier is unlocked, but only for configuring it below its standard ratio. On Athlon 64 FX you can go both ways: up and down.
First we increased its HTT clock using its standard clock multiplier (14x). We could achieve only 209 MHz, making the CPU to run at 2,926 MHz internally, a 4.5% increase on the CPU internal clock. Over 209 MHz the system was unstable.
Then we increased the clock multiplier to 14.5x, making the CPU run at 2.9 GHz, a 3.57% increase in its internal clock. We couldn’t increase HTT, the system was always unstable.
Then we played descreasing the clock multiplier to 13.5x. At this configuration we could put the HTT clock at 216 MHz, making the CPU run at 2,916 MHz internally, a 4.14% increase on the CPU internal clock). Over that the system was unstable.
We didn’t play with voltage or with the cooler. You will probably achieve better results than us with time and patience.