Building a new system,Opinions please.

[Xenosaga]

makes no sense to force anyone to do math like that? A psu labelled 400w will provide minimum 400w of power.
Also, the 280x uses 380watts at peak levels and I don't think any 400w psu is going to cover that - even with a 580w gpu you'd be pushing it.

Anandtech measured 393W for the whole system at the wall in Furmark (GPU stress tool, much more taxing than a game). Again, it's an overclocked GPU and overclocked CPU. The unoverclocked 280X goes for 360W.
A good 500W PSU would suffice easily for such a system. 550W if you're paranoid.