I have done a few calculatings how much faster the Nehalem will be at the same speed.
I have make a excel file,but i don't know how to put it here online,maybe someone can help me with this!

Here is a +/- speed up i get if i can overclock the Nehalem :

QX9650 @3.6Ghz -> +41.34% for Nehalem @3.6Ghz
QX9650 @4.0Ghz -> +46.40% for Nehalem @4.0Ghz
Skulltrail @4.0Ghz -> +27.95% for Nehalem @4.0Ghz
Skulltrail @4.2Ghz -> +20.44% for Nehalem @4.2Ghz

JP.

PS. This was with a Nehalem (ES) ,so these are the min. differences speed up you get!