No solid data.
A lot of this depends on how many words you are dealing with. A single word search will see a bigger difference between Overture & Google. A multiple word phrase will see a smaller difference between the two.
But how many surfers are going to check out the number 1 listing and how many are going to visit other sites in the top ten?
Overture is a good guide to how people use engines, but each month is gonna be different also.