install the rdsnotidy widget, it includes the power boost for the fm transmitter, as well as displaying useful stuff on the radio such as texts as they arrive etc.

as for a frequency, just find a clear one on your radio and tune the N900 to that.

oh, and i find that the i getg no interference whilst charfging with a branded nokia charger, until I start 3g data, then i get some noise unless i unplug the charger.