The OP will be fine with the 400w PS.
The motherboard only supports four SATA devices... and it's a Cube mini-ITX build... so there will be no more internal drives.
Regarding the video card:
The onboard 4000 series graphics do not work well with video editing apps. New Blue Titler Pro (used with numerous video editing apps) won't work at all...
Titler Pro works with a HD5450... and it (of course) runs better with a video card with more speed.
Based on the card the OP spec'd, I'm guessing he has video editing in mind.
One of the faster passive-cooled video cards that's readily available...