HT is Intel's technology to run two threads on the same CPU *simultaneously*, sort of like a 2-CPU system. However, HT is not that magic, it doesn't really give you the same performance as 2 physical CPUs.
By using HT, although you have 2 virtual CPUs now, each of them is in fact slower than the original CPU when you turn HT off. This is especially true for some type of programs, like IO intensive ones.
So if your workload is mostly single-threaded, you may want to turn off HT if performance is important. To make it simple to understand, for a 3GHz P4, you have two options:
1) HT off: it'll be a single 3GHz CPU
2) HT on: it'll be two (roughly) 2GHz CPUs
Whether 1x3GHz or 2x2GHz is faster really depends on your workload and personal taste.
