Google HD6870 and TDP. You see the MAX power the card will draw is 151W.
"a8 quad core 8 gig ram 1tb hdd and a sapphire radeon 6870 " Plenty. A 400W PSU would be plenty.
If this is a system you are building then A8 APU is a bad processor choice to go with the HD6870. All of the video will come from the 6870 (great card) all of the A8's video circuits are wasted (who care) and all of the trade-offs made to get both CPU and GPU in same chip mean you have a slow expensive CPU (bad). If you want AMD pair the 6870 with a phenom X4. If you are willing to go intel an i3 will kill the a8 and more than match the Phenom.