While calculators give you a ballpark, most of the calculators provide an over-inflated requirement. I highly recommend posting your proposed system here to get some advice from forum regulars and doing some research...read reviews and articles on PC power consumption. Modern PCs use much less power than most people think. Don't let lack of experience, great PSU marketing, and bad recommendations on forum boards influence you to get way more PSU than you really require. Read this for some great info on how much power modern PSUs really use: http://www.silentpcreview.com/article1245-page1.html
Edit: ct1615 I just re-read my post - I was NOT saying that your recommendation was bad. I was warning against other inexperienced recommendations that we see on these boards...750W PSU recommendations for single-GPU PCs.