USB devices can pull 10 Watts a piece, and we aren't just talking raw power. If they use the 5v rail, they easilly pull 5 volts at 900 mA. Your Narwhal build is rated at about 490W under 90% load without capacitor ageing. Which means that, after 5-6 months with 4-5 reasonably demanding USB devices (headsets, mice, scanners etc), the system will become unstable, and unless you're in the know, people usually jump to replacing memory etc rather than considering the power supply.
All I'm saying is that I would definitely pay another $20 for the higher end PSU than buy the lower powered one to begin with. If anything, simply so it gives you more overhead for expanding (i.e HDDs, SLi/Xfire). Once you're spending more than $1000 on a PC, you might as well spend an extra $100 tightening all the bolts and making sure it's going to function at it's maximum.
---------- Post added 2011-05-09 at 07:53 PM ----------
Why would you use the preface that the person isn't being smart and then immediately say something very un-smart yourself?
An atom of Uranium is an atom of Uranium. It's one atom out of 6.022x10^23 needed to make one mole of Uranium. Uranium is an element, but that doesn't mean that an atom of it isn't also the element Uranium, it's just a very, very, very small amount of it.
Maybe you're confusing allotrope's and elements. Atoms aren't mix and match - X number of atom A doesn't make element Y while Z number of atom A makes element W - an atom of one thing has a specific amount of protons, neutrons and electrons. An isotope of that element has a different number of neutrons, and an allotrope is a different arrangement of those atoms (i.e, graphite vs diamond in the case of Carbon).