+1 to eratu

I have it enabled, it will save you power. Doesn't affect performance.

According to the bios on my Dell laptop, disabling the speedstep puts the processor into LOW POWER mode, not high power.

There's a good article here on the power settings for XP, so leaving it set to 'always on' is disabling speedstep anyway.

http://www.orthogonalthought.com/blog/index.php/2007/09/intel-speedstep-windows-xp-and-confusing-power-profiles/

I can tell you that with the P5B, until I went into the BIOS and turned this off (again it's not called speedstep in there - it's something else), my CPU speed was not reading the 2.13Ghz it was supposed to be - it was more like 1.6ghz. It didn't matter if I was actively using the system, or even stressing it with Sonar, it didn't switch to the "faster" mode like I would expect it to when "under load". So I nix'd it. I saw an absolute change in the amount of CPU being used on the Sonar meters to run the same project, as did the subsequent slight overclock. As mentioned, this is on a Core2Duo, not a quad, but I can't imagine that running at a lower CPU is going to work BETTER than at the highest for the chip (not overclocked, but rated speed).