Please bear in mind that ALL of your peripherals and connections add up to your power usage. So PCI cards, keyboard, mouse, USB devices, optical drives, storage etc. However a i3 3240 r7 260x build will likely pull a load wattage of around 300W, so a 400W should be fine. But may limit future upgradeability. I have based the calculation on the following build:
i3 3240
R7 260x
1x 4GB DDR3 module
Wireless PCI card
Several USB 3.0 devices
Gaming keyboard
Gaming mouse
2 x 140mm fans (intake and out)
1 7.2k RPM SATA HDD
Optical drive