HTT = a important base system frequency for a64 systems. its setting is a varaiable in just about all the main frequencies for that socket motherboard.
Basically every amd 64 motherboard has several settings
cpu speed = HTTXcpumulti
memory speed= HTTX(memory divider)X2
hypertransport bus speed= HTTXLTDX2
my system has 260X9=2340
my memory speed= 260X(5/6)*2=433
hypertransport bus speed is 260X3X2=1560
HTT is a base frequency for many of the things in the system on a a64 system. As you can see HTT plays a role in the outcome of the cpu, memory, and hypertransport bus speeds.
On socket 754 you should keep the hypertransport at or below 1600, while on the socket 939 systems you should keep it at or below 2000. That means the HTT should be either 200X5, 250X4, 300X3, etc.