A 550 watt or a 600 watt would do just fine. I personally love the Corsair TX 600, and that would be great for his build. If you want to go cheaper you can use the CX 600, which is a lower quality PSU, but is still a good quality one from the same company.
CX600 is a cheap power supply with bad capacitors.
I recommend you go with the xfx 550 watt