A 550w is fine for those cards, but i would recommend a reputable brand such as XFx or Seasonic. As for the graphics cards, the 670 is slightly better at 1080P ( but costs quite a bit more ) and the 760 is better at higher resolutions ( due to the higher memory bandwidth ). However, the 7950 is on sale for 240 USD at Newegg, and I would recommend that. Also, it can be OC'd to 7970 performance levels:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814202026