If you'll be sticking to a single GPU, a good 500W-600W PSU would be sufficient for your build. A quality modern PSU has full range active PFC (no little voltage switch) and some level of 80+ certification for efficiency. Seasonic, Antec, Corsair, XFX, and Enermax/LEPA are among the better brands.
If you think you might run two GPUs in SLI (nVidia) or Crossfire (AMD), you will probably want 650W-750W; I would suggest you go to the PSU sizing calculator at http://www.extreme.outervision.com/psucalculatorlite.jsp
and play around with the specifics of your build.
If it's the only drive in your system, make sure that 2TB Seagate is not a "LP" (equivalent to WD "Green") model with a reduced RPM, else it will be slow.