Hyperthreading is good in theory, bad in practice. My 3 Ghz Northwood with hyperthreading was about 1/4 as good at multi-tasking than my Opteron 170. If I had 1 process taking up a whole "core" on my northwood, I couldnt do anything else. On my Opteron, I can do anything I want with that extra core. Night and Day.
Hyperthreading only emulates two cores, it doesnt actually let you run 2 processes at the same time at full speed.