The power supply you have is adequate for your system. You really don't need a new PSU.
AX series (most expensive) is 80 plus platinum certified
HX and RM series are 80 plus gold certified
HCG is 80 plus bronze certified.
These are all good ratings, but gold is more efficient than bronze and platinum is more efficient than gold.
80 plus efficiency from wikipedia:
http://en.wikipedia.org/wiki/80_Plus
If you are looking for a high end power supply like these, you should consider Seasonic. They are widely considered the best on the market.