I created a quick CPU benchmark using PC Wizard 2008 (I'm too lazy to download the latest version) on Windows XP comparing CPU performance between Hyperthreading enabled and disabled.
Hyperthreading Enabled:
Whetstone (x87 - Float.) : 2277 MFlops
Whetstone (iSSE3 - Float.) : 4297 MFlops
Dhrystone (iSSE3 - Int.) : 7904 Mips
Computation of "Mandelbrot" (iSSE3) : 38.625 sec.
Hyperthreading Disabled:
Whetstone (x87 - Float.) : 2281 MFlops
Whetstone (iSSE3 - Float.) : 4302 MFlops
Dhrystone (iSSE3 - Int.) : 7916 Mips
Computation of "Mandelbrot" (iSSE3) : 55.343 sec.
