Hyperthreading is like having a duel CPU set up sharing the same front side bus. Not exactly thrilling to the everyday user. But handy in some multi tasking areas.
HyperTransport is a point to point bus made by AMD for their new K8 CPU'S (Opterons and A64's). Meaning there is no middle man anymore. The original FSB goes like this: CPU to the motherboards north bridge / memory controller then off to the RAM. HyperTransport is a direct link from the CPU to the RAM. So it's Just the CPU to the RAM. Hence, Point to Point. The opterons have their on onboard memory controllers so it no longer needs the middle man on the motherboard.
This is one of the reasons why the Opteron has been so successful in Duel and 8-Way Set ups.