At 720P you should be able to play most games, your CPU will just limit you in big games like Civ, GTA etc. At 1080P you will probably have to lower graphics settings on most games.
I'd build something like the one BadActor suggested although that only needs like 300W+ so I'd get a good quality but lower wattage PSU with it like the Antec EA380 or XFX 450W or something