let me clear things up a bit for you...i made images to help you out...
when they make the silicone the purest silicon is in the middle (If you want to know more about the process look it up, i dont feel like explaining)
Therefore the best cpu's are in the middle.
Here is a wafer of cpu's, they come out of the plant like this then they cut out the cpu's from the wafer.
http://img529.imageshack.us/img529/5990/300mmwafer.png
Like i said the best cpu's are in the middle the next image i made was an image showing out of all the cpu's wich one are the 920's 950's and 975's.
http://img529.imageshack.us/img529/3492/300mmwaferj.png
So, as you can see maybe out of 100 chips only 5 are 975's.
These will operate on lower voltages with higher frequencies. For instance if i had an i7 920 then it may require 1.375v to reach 4.0 GHz but if i had a 975 then it may run at the same speed with 1.25v
If i were you i would get the 920 and stick with the current motherboard you have picked out. Because that motherboard has VERY high end capacitors that allow for more stable overclocks.
If you look most of the enthusiasts buy buy the EVGA X58 classified...this is obviusly out of your budget so i would go for the standard evga x58
The 920 will have a very substantial overclock of somewhere around 4.3 Ghz which is 1.6 times as fast as it originally was
but if i had a 975 the max overclock ON AIR may be around 4.6 GHz wich is 1.43 times as fast as it originally was... The 975 is probably well out of your budget but the 950 will usually offer an overclock close to what the 920 was so that would be somwhere around 1.43 times as fast as it originally was.
Thus that brings us to our conclusion that the 920 is a better buy
EDIT: Don't bitch at me if you claim you got higher overclockes with these varius chips i am just setting a standard