You'll need to replace the power supply, I don't think that unit currently installed is a top class part and 500Watts is cutting it fine for a GTX970, and isn't really enough for a GTX980.
I'm not convinced a FX6300 CPU will drive a GTX980 to its full potential so for that reason I'll suggest you aim for a GTX970, or the R9 290 (bit slower, bit cheaper, uses more power).