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I have a Core i7 920 and for get more speed of emulation i have to turn off the hyper-threading? Which is the percentage of speed gain without overclocking?

Results from turning off Hyper-threading will vary.
Between users,
hardware,
settings,
and probably even different games.

You should be testing the same scenario with it on and off.
You tell me what happens for you.

My bout with an i5-650 dual w/ HT (disabled) showed serious gain in FPS stability and performance.
I've also seen other with the same, or similar experiences,
as well as those who do see benefit from keeping Hyper-threading on.

Three things are certain, though;

• Results have a tendancy to vary, especially with PCSX2.
  
• Hyper-threading does increase running temperature, and if you're not using the extra threads, the additional temperature would be unnecessary (and reduce OC potential).
  
• Intel's Hyper-threading revisions will improve things with time, as they already have.
  

You will have more gain from the overclocking you can achieve with it off than having it on, but in general it doesn't do much for pcsx2 at all

like guys already said it varies from game to game, but from my experience it's mostly beneficial to keep HT on, but if you're buying CPU for gaming pc HT definitely not worth extra $100 not for gaming.