Freebie, Upgrade, or "In App" purchase?

Beat the Boss is getting closer to finalizing, but there are so many details it may still be longer than I hoped till I hit the finish line. The timing of the game is good because the economy is bad, people will be pissed for a long time. The subject of hitting your Boss is nothing new, I am providing a 2010 delivery method. AT&T is running at 300 percent because of Smart Phone usage. The size of the app is an issue. I know for a fact that being under 10 megs will be essential for the free version. 10 megs is the limit for "over air" downloads. Previous apps I have made were too large and people gave up if they weren't at a Wi-Fi spot. My free version will have an option to remind yourself to buy the full version with an angrily written e-mail sent by the Boss. I have had thoughts of selling new levels as in "in app" purchase. From what I have read, people see a short sales spike right after you upgrade. Being my game is level dependent, I can keep adding levels indefinitely. Now my thought is, how often?