WD Documentation [Edit:] Check out the power consumption figures. You really don't have to factor HDDs into PSU choice unless you have 10 of them or something.
For power, unless you fold or something like that, I doubt your computer will draw more than 80W (from internals) in normal operation. My old home server with Core2Duo E6400, no GPU, and three hard drives drew about 65W from the wall idle, and ~110W starting up. This was with a GUI-less Linux.
If you don't pick an overkill GPU with PCIE supplemental power, you'd be fine with a *good* 300W power supply if it has enough cables. A bad PSU (like the ones Hardware Secrets / JohnnyGuru say are just within spec or out of spec) will periodically kill hard drives.