For me, I would want to "play it safe" as they say and buy a bigger PSU than actually needed - so, say for example you're like me and want to get Sapphire's R9 TOXIC 280X, which for all intents and purposes - plus noted in seemingly every review I've read about that card in particular - draws a bit more power than a typical R9 280x. Sapphire recommends a 750 watt PSU to run that card, a 1000w if planning to run two of them via Crossfire. I realize they're probably just putting it out there as a way to CTA (Cover Their Ass) but why take a risk? I sure as hell don't want to, especially for that kind of money, so that's why I'm going to get a Seasonic X850 just to "play it safe."