Quick and dirty calculations of power usage (min - max):

  Motherboard: Asus Maximus VI Gene                   - 57 - 123
  CPU: Intel Core i7 4770K 3.5 GHz Haswell            - 70 - 80
  RAM: Kingston HyperX Beast 4 × 8 GB DDR3 2400 MHz   - 12
  SSD: Kingston HyperX 3K 480 GB (×2) (Striped)       - 1 - 4
  HDD: Western Digital WD Black 3 TB (×2) (Mirrored)  - 16 - 20
  GPU: Radeon R9 280X                                 - 15 - 257
  Cooling: Corsair H80i                               - 4
  Fans: Noctua NF-S12A (×2) & Noctua NF-F12 (×2)      - 4
  Sound: Bowers & Wilkins MM-1                        - 12
  Other: Bluetooth LE & Wi-Fi PCIe Module             - 2

Final output: 193 - 518 Watts. 750W PSU is not a huge overkill, especially if he wants to add more drives and/or a bigger GPU. Also as mentioned by others here, PSUs generally run quieter at lower loads.