In my casual micro-benchmarking experience, hyperthreading on my Core i7 920 can be worth about 50% extra. That is, the difference between 4 threads and 8 threads on 4 hyperthreaded cores can be about 50% (and I can see that Windows 7 is scheduling the 4 threads across the 4 cores correctly).<p>I expect that if the code was extremely tight with no cache misses, any extra gain from hyperthreading would be pretty limited, if not negative due to extra thread switching costs.