It is a little subjective. Some people like to play it very safe and others don't mind the maximum power draw reaching 80-90% of the PSU rating.
I would say play it fairly safe, let's keep it at around 75% of the rating, so a 550-600w PSU would be great. Pay more attention to the reputation of the brand. Corsair for instance is a reputable brand, basically stay away from extremely cheap PSU's even if they say they are 800w or something. There is a reason why they are so cheap.
Hope that helps dude!