550w is plenty for a single GPU system even with the power hungry 7970 GHz.
If you are planning on adding another 7950 for 2-way Crossfire in the future though, best get a 750w unit.
Also, I don't believe NZXT has any reputation be it good or bad so far, so I wouldn't get it for now. Better safe than sorry so get the known brands.
You can also do better on the SSD. Get a 256gb Samsung 830 or Crucial M4.
Vertex 4 is what I would usually recommend but their pricing is a bit retarded lately.