OK! I Really think that the V8 is terrible for anything above 3.8, to be honest at full load (prime95), Although it actually does keep my 3.7 955 cool at idle like blackhawk said, When you go full load its hot. But your best bet if you plan on getting it to 4.0ghz or 3.9 ghz is to get the Thermalright Venomous-X first suggested.
Here's some benchmarks on the Venomous-X, btw it beats the Prolimatech Megahalems, AND if you do get the Venomous-X remember to buy two high CFM fans for push-pull configuration for the best results.
Great review in my opinion, sorry by other forum, http://www.ocxtreme.org/forumenus/showthread.php?t=5576