Power output is limited by the modem so a beefier power supply will not get you more RF power. The supplied antenna is actually pretty good, about as good as you will get with an omnidirectional dipole antenna. Although plastics do have an effect on RF performance and the antenna patterns they are minimal compared to any metal (including the Nova itself) so keeping your antenna away from large metal planes like filing cabinets, metal boxes, etc will keep you from being blocked or have reflections effect the quality of the signal.
Your next option as you mentioned is a directional antenna, Note these might violate FCC requirements as I believe you are limited to a peak directional gain of like 3-4dbi. Also be careful of antennas that have or require long RF cables. Having a 6dbi gain antenna with 15 feet of crappy RF cable will probably perform worse.
Also only look at antennas that have a datasheet with antenna patterns, random crap of amazon / ebay / etc will likely do no better than the provided antenna.
Lastly note that monopole antennas need a ground plane to work effectively (they need to be mounted perpendicular to a metal surface with the metal surface shorted to the jacket of the RF cable). Otherwise these are also pretty junky.