The Antec P280 is a fine choice. Even with stock fans (set on low), you'd probably be perfectly happy... at least for a while: Quiet can be addictive... and quieting a computer can become a serious addiction.
The PSU could be quieter. IIRC, that Corsair is made by Channel Well, and it isn't anything like the Seasonic-made ones -- considerably noisier even at min load, default fan speed. Cheap solution is to swap out the fan, if you are a DIYer. Replacing it with a PSU from SPCR's A-list will get you a higher efficiency, quieter unit.
The WD Black isn't quiet either, though, and it might even be loud enough to make the PSU noise a non-issue -- ie, even if the PSU was quieter, the HDD might be loud enough to make no (or little) difference to the total sound. It probably needs suspending & enclosing to be really quiet, depends on how far you want to go. Maybe the rubber grommets in the P280 case (and other cases) might be enough for you.
Another option is to move to an SSD for your OS & programs -- a good 120gb drive is pretty cheap these days & big enough for just about anyone. Move the WD to an external eSATA box, use long cables to distance it from you, and use it only for data, you could even keep it turned off and turn it on only when you need to access it.
Hopefully your video card is quiet at low load and its fan doesn't yoyo in speed with load. If so, consider a heatsink replacement.
Finally, the CPU is really the easiest thing to cool quietly these days -- if you have a big cooler with a big fan. If you are using a stock HSF, again, I refer you to the recommended products for a replacement -- http://www.silentpcreview.com/section5.html