Is a 420 watt power supply good for a Amd 7850 2gb

When I see "420W," I think Apevia / Aspire, and their units are maybe good for 250W, so absolutely not! OTOH, a 380W Antec Earthwatts, or 450W Antec VP-450, 450W Rosewill Capstone, or other quality PSUs around 380W-450W would be fine for your build.