or set up an account and it shows you projected impressions/click throughs etc etc.
Or find a really nice Google rep (loads around here) and ask for an inventory report, assuming you are going to spend lots of $$$s on Premium Listings.
From past experience Google traffic is averaging 200% of Overture figures (dependant naturally on industry) but this is a close guide to work on.
Shak
The problem with the Overture tool is that you never know what % of those searches are SEOs, PPC managers or competitors.
there again you dont know that with Google either :(
best of luck.
Shak