I don't say it to spend money, but to try and keep the investment as safe as possible. Yes, the power consumption is probably not going to exceed around 450 watts, but there's other factors to consider like overclocking. That adds a lot of voltage. What about future upgrades? Another factor to consider. Also, power supplies don't actually deliver the amount that is stated. In the 650 range, unless it's silver or above, it's going to lose about 5-10% of the actual power supply limit. So, a 80+ 650W power supply is still going to be around 550-600W or so in the end.
If the poster didn't have high end equipment in the PC and it was simply a $500-600 build then I would have said a lower/cheaper power supply was ok, even if it was barely getting the requirement. But, when you have a 4770k and a GTX 780 in your computer, I think it's a good idea to get a power supply that exceeds the limits, just to be safe. There's also a price differentiation to consider. The difference in price from a 650W power supply to a 500W or 400W power supply is about $30-40 at most. If the poster can afford a 4770k and a GTX 780, then it's safe to assume that the poster can afford to get a 650W power supply. I try to recommend based on the actual build and what the poster limits in budget.