Thanks for your input. The 4 sites have been launched recently, so our user base is quite small at the moment! We are getting feedback from other channels too, but we think getting comments/advices from geeks is quite valuable (especially regarding the use of AdSense, the Amazon associates program, and the concept in general), even if those comments are short and simple.

And I guess your strategy for ads placements will probably be more relevant later on, if the traffic ever justifies it.