Marshall-Mathers wrote

r00t wrote The 780 is relatively light on power and the rest of your system is as well, so a good 550w unit would be ideal.

It is. However, for $10 extra, it makes sense to leave an extra 100 watts for future upgrades.

I can't think of any upgrades that could make use of that. Adding drives, fans, sound cards, etc. shouldn't be an issue for a 550w unit. I'd take the $10 and my better efficiency.