If you search the forums and repos there are instructions and even a packaged app to boost the FM transmitter on the N900 to fill a whole room.
If you take most external FM transmitters you can jumper resistors or even overvolt the power to the amplifier chip, sometimes you need to add an antenna cut to length http://www.qsl.net/kb5wck/antenna.html
for the frequency you choose maybe also jumper a attenuator on the way to the antenna and you can get a few hundred meters of range on an open FM channel just watch out for the FCC.