An omni directional antenna for what purpose? Mobile? Stationary? WarDriving? Wireless pen testing? Define the use, and someone may be able to help you chose the right antenna.
As far as the "power booster" goes: Radio Frequency Power Amplifiers (the real name of such devices) are something that can be dangerous if you don't know what you are doing with them. RF radiation burns can be serious, and it's pretty easy to get one with an amp, especially if you don't know what you are doing. Give yourself some time to become knowledgeable in the subject before purchasing one. Amps have their applications, but there are distinct downsides to using them. For example, besides boosting the RF power, amps also boost the Noise, so you have to be prepared to deal with that. Amps also increase the potential for SWR damage to the equipment if you cables aren't up to snuff. Another problem is that it is very easy to exceed the legal limits using the wrong combination of antenna and amp. think of it this way: Do you really need a visit from the FCC and a $10,000 fine (or worse, endanger you health or that of your family) because you didn't know what you were doing?
Once you have gained some knowledge of the subject, then you can determine if you even need an amp.