HT is an effort to keep the cpu busy, as in busy busy. Even though a cpu may show 100% useage, in actuality it might only be 30% _efficient_, instruction execution wise. This is because sometimes threads must wait on other threads, an os call, or i/o, to complete the function, thus leaving parts of the cpu idle. With HT the cpu is kept "more busy" by keeping the registers humming on two threads rather than on one single thread. Threads being basic cpu food. Well ok, threads are the actual parts of an application that gets processed by the cpu. Consider HT to be vestigial cpus rather than literal cpus, even though the os considers HT to be SMP. In a single cpu non HT system, only one thread can run at any given time. On a Symetric Multi Processor system, or HT enabled cpu, two or more threads can be run concurrently.
The gist is that multithreaded apps can see a 5 to 30% boost in performance, and single threaded apps can see a benefit as the os can do something on the cpu at the same time as the app thread. This generaly means a smoother system. But not always; sometimes HT makes an app perform worse, as certain amounts of resource contention can occur in the cpu, creating delays not present in a non HT cpu. I'll take that risk.
As you can see, dual core is the way of the future. HT is kinda being left behind, but is still worthy. The Prescott ver of the p4 has some slight enhancements that the earlier HT p4s don't have. Still, a p4 with HT enabled is better than a p4 without, no matter what the ver.
The guts of it all can be found at arstechnica and other forums, as well as at Intel.