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If I enable Hyper-Threading for my netbook which has an Intel Atom (1.6 GHz) will the kernel see two virtual 800 MHz processors?

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You seem to be confusing clock speeds with processing speeds, as if an "800 MHz processor" was a sensible term for a processor that had half the performance of a processor that happened to be clocked at 1.6GHz. This is a common misconception, but it's a misconception nonetheless. If you're asking if the virtual cores will have half the performance of the physical cores with HT off, then ask that. But you can't use "800 MHz processor" to mean "a processor with half the performance of a 1.6GHz processor" because performance doesn't relate to clock speed that way. – David Schwartz Oct 15 '11 at 1:38

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No, it will create two virtual 1.6 GHz processors. (However, when not under load, they will clock down to a much lower clock speed, then 800 MHz might be correct.) Do

cat /proc/cpuinfo

for information about them.

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I plan to install kernel26 with bfs and hyper threading enabled, and will try it out. I hope it works well! – Google Sep 29 '10 at 6:39
They generally do not clock down under load ( unless overheating ), but rather, they share certain resources, so if they are both busy and using those resources, one has to wait for the other to finish with them, slowing things down. You are still usually better off than without hyperthreading, just not as good as actually having two fully independent cores. – psusi Aug 4 '11 at 14:24
@psusi He said they might clock down when not under load (EIST). – Chris Down Nov 24 '11 at 17:27
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My experience is that by enabling hyperthreading you do not get double the performance. But you do get around 1.5 times the performance if you can utilize the 2 CPUs. So to put it in your language you get 2 CPUs running at 1.1 GHz when 2 CPUs are needed and 1 CPU at 1.6 GHz when only one is needed.

In other words: I have yet to come up with a single situation where enabling hyperthreading will slow down anything.

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