The HTT is the link between the processor and the motherboard chipset. In socket 754 systems, this was 800mhz (1600mhz) which is the 4x option. In socket 939 systems, this was increased to 1000mhz (2000mhz) as the 5x option. As you can see, the multiplier option correlates with the processors FSB speed. So when you overclock the FSB (to increase the speed at which the processor communicates with the RAM), the link between the motherboard chipset and CPU is also overclocked. So to avoid this being the stumbling block when overclocking, it's usually best to drop the HTT to a lower setting. Doing so has literally no effect on the overall speed of the system because the link is already much faster than it needs to be.