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   I am thinking of getting an Intel i7 machine with Hyperthreading. Does Hyperthreading speed up fractal rendering and by how much? How much does hyperthreading  speed up Ultra Fractal, Mandelbulber and Mandelbulb 3d? Thank you very much for your answers.

The most important speed up is the number of cores - double the number of cores will essentially double your render speed.
Theoretically hyperthreading should make each core able to run two threads at full speed, thus doubling your render speed, but in practice double speed is never achieved, e.g. at a rough guess 2 cores hyperthreaded will be equivalent to 3 cores without hyperthreading rather than 4 although this may vary from one chipset to another (and upon the code being run).
Of course the most important aspect is that the software used actually supports multi-threading in the first place, Ultra Fractal does, and I think Mandelbulber and Mandelbulb 3D do too, probably also ChaosPro and Xenodream, I'm not sure about Apophysis (at least not all versions) or Chaoscope.
In Ultra Fractal you need to make sure that the "min threads" option is set to match your hardware - e.g. use 2 for 2 cores or 1 hyperthreaded core, 4 for 4 cores or 2 hyperthreaded cores etc.

I am tempted to challenge Hobold to a buzzword duel, but only we would enjoy it. It would also not add to the discussion since I agree with his analysis. I can however provide data on core scaling.

In brief mandelbulber 0.90 linearly scales with number of cores (as DM stated). It actually is a little better than linear. This is usually attributed to parasitic demand prefetching. Basically if all cores are going to need a cacheline, only the first core pays the price of going to DRAM. All praise to the Buddhi because this does not happen by accident.

I cannot provide current measurements on hyperthreading. From previous experience I can say that highly parallel floating point code often sees a higher than average speedup. This is because the FP operations tend to involve data dependent chains of  long latency operations which use pipelined execution units. Thus multiple execution chains can overlap with minimal interference.

I am really interested in seeing more real data. Can someone out there who has a hyperthreaded processor try the experiment. You should be able to turn off hyperthreading in setup at boot time (only try this if you have another computer to go on line to figure out what you screwed up.) Time a series of standard renders with and without hyper threading.

Raw data: Mandelbulber 0.90. gcc-4.4, 2. dual chip quad core Zeon X5355 @ 2.66Ghz. Initial start Mandelbulb power 8 settings. Average render times in seconds.

   image     1 thread/chip    4 thread/chip  speedup/thread
 500x500          18s                     4s               1.125
  1kx1k             80s                    19s               1.05
  2kx2k            361s                   87s               1.04

It is interesting that the cost per pixel is going up with size.