It's generally agreed throughout this forum that a Pelt can be air-cooled if it's rated for less than 80W transfer.
You need to lookup the Power dissipation of the CPU you use, then add (at least) 25% to that number. That will give you the required Pelt rating.
Of course the more powerfull, the better
You can then expect your CPU temp to drop below freezing (at idle) and stay around 10C at full load. This of course means that you'll have to isolate it for moisture from condensation. See Swiftech's site for more info.
Of course if you want to get fancy... you do what I'm fixing to do: make (buy) a variable power supply that will keep the CPU at the highest temperature that it can be cooled at (i.e. at full load).