the power gets sucked up a lot by the video card and the cpu, so you want something thats powerful to supply all the power it needs, plus a little more for headroom. 750W should be enough, but if you ever decided to go SLI, you'd probably want to look for an upgrade on the psu.
Um... no. Just no. That system is a 400W system Max. 750W is only needed if he were going to SLI, and you don't SLI 670s to game at 1366x768