If you get a GTX 560 or higher you will need a 550w PSU Any thing under that you will at least a 400w PSU on the AMD side anything over a 6870 you will need a 550w PSU and any thing under you will need a 450w to 400w.
All this is recomemded by both nVidia and AMD. The brands should be Corsair, Antec, Seasonic, XFX, Silver Stone, and some OCZ's. I will link you to a PSU chart. Use the Tier 1 and 2 spots for picking brands.