I would not say a 400w would be good. I personally prefer having over kill for my PSU because as they age they can't put out as much power. Plus having more then enough power now lets you expand with more stuff in the future. Like a USB 3.0 expansion card, more HDD's, ect.
Ran the specs he gave (with some guess work like 4 gig of what kind of RAM, I guessed DDR3). It gave a number of 237 watts.
Now as for future expansion, I added another HDD, a (currently) decent video card, and a PCIeX1 card to represent a USB 3.0 upgrade. I also took into account the fact that USB 3 puts more power on it's bus then USB 2 so I up'ed the number of USB devices from 3 to 6. The final output was 319 watts.