Amd radeon question

I am waiting for my gpu to arrive and I would like to know how many watts I need for my system.I have a pentium dual core 2,5ghz 2gb of ddr2 ram my motherboard is an ga g31m s2l and the gpu I will buy is the radeon hd 6670 1gb ddr3.How many waats do I need.I currently have 500 watts and my current gpu is the nvidia 9500 gt.I am asking this question again because i want a clear answer.I dont know much about psu things but a sticker says -5v and 0.5a i dotn know what that is.