My comments:
Find out what the regulations are for your country/area. Determine how compliant (or not) your transmitter is in it's present state. The IC manufacturer should be able to advise on the output power-most likely it will be into a 50 or 75 ohm load. Regulations may be in terms of output power or input power to the final amplifier and there may be limitations on the antenna. Sometimes the regulations are such that distance or property lines are the governing factors.
In order to get the most out of the power you have the antenna must be matched to the transmitter. Amateur radio publications cover this nicely. A dipole antenna can be adjusted to the right length to match so that maximum power is radiated. There are various vertical, yagi, and other antennas that use techniques to focus or direct the power so that what little you have goes where you want it to go.
Before adding an amplifier take a closer look at your schematic. There is a change that can be made however you might need to be more certain that the antenna presents a proper load to the transmitter. The schematic may also be the way it is because of regulations.