Hyperthreading basically makes two fake cpus out of one real one, whereas dual core is two physical cpus on the one chip.  The basic difference is that a hyperthreading cpu shares the cache and bandwidth between the two threads, dual (or quad) core should have separate caches.

In most applications, you get a roughly 45% increase in speed when Hyperthreading is turned on, as opposed to it being turned off.  The dual core speed increases I've seen posted for B6 are about 80%.

A dual core machine with hyperthreading would be able to run 4 threads at once (but not as fast as a quad core).  This should still be a significant increase in speed over a single core/non-hyperthreading cpu (like mine).

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