I'm a little confused Finndrummer, in one post you say
am running at stock speeds and with eist off my multiplier hits always it's max "x24"
in another post you say
and from my experience, eist must be enabled to have turbo boost working.
but if your running a i5-750 then 24x is a turbo bin so turbo boost must be enabled for it to work. If you load all 4 cores with something like Linpack then I doubt you will see 24x but probably something like 21x.
Tatty_One, from what I've personally seen, which isn't that much, I can believe you when saying it works that way with many boards.
Your system specs are an i7-920. The i7-920's non-turbo ratios are 9-20, turbo bins are 21-22. What happens if you set 21 in the BIOS with EIST disabled? If you get 21 then your BIOS has enabled turbo with EIST disabled. You might not see 22x as the BIOS might cap turbo to 21 in this case. Finndrummer, you could try this also, 21x with 4 cores IINM or even just enable 2 cores and select 23x or one core and 24x. I'm not sure if they are the right maximum multi's for the 750 with 4,2 or 1 cores but something like that. They are however turbo bins.
Now I'll try not to be too confusing here, although that might be difficult

, but after doing this if you run RealTemp and open the settings window you will likely see that EIST is enabled. WTF. This is because instead of really disabling EIST your BIOS is setting the multiplier then not passing any performance states to the OS so the OS should not try to change the multiplier which is effectively like disabling EIST.
If your BIOS has not locked EIST the box will not be grayed out and you could go ahead and really disable EIST. It will not affect turbo boost but will leave the system at the last multi that was set. If you have that 21x multi running try ticking the "Disable Turbo" box in RealTemp. Then you should see your 21x turbo multi drop to a 20x non-turbo multi. Turbo really has been disabled.
Another thing you could both try is to enable the 20x multi with EIST disabled in the BIOS and then un-check "Turbo Disable" in RealTemp if it's enabled and run a single thread such as SuperPi while keeping everything else generally idle to see if that turbo pops up.
The "i7-Turbo" application that comes with RealTemp is real good at seeing changes in your multipliers ratio.
C-States are not linked to EIST, they are driven by the OS and/or sometimes software. Turbo boost however, is linked to C-States.
IMO if your using a fixed core voltage then your probably not going to notice much difference in the power usage average with EIST enabled or disabled.