Cooler master Gx500 have issues concerning efficiency...Even if it is 70% efficient, its more than what you need.
Since 7870 will not use more than 300 watts ( ur entire PC combined), Its safe to use your current PSU...
I have used 7950 on my Cooler master 460 watts PSU for 1 whole week..
SO Ur PSU is adequate to run 7870.
Check here..
Hardware Journal | get the news - Club3D HD7870 Royal Queen
The power consumption of entire system is upto 260 watts only..