I LIKE the "twice the wattage" but have to write that sometimes that's not enough. (say what?) There are some Quad rail PSUs that have awfully low amperage ratings on the +12V. To avoid this my generic PSU spec is "twice the wattage" and a single +12V rail.
That seems to keep folk out of trouble.
Now you might ask why double. My answer is that the PSU rating is for a new PSU. Since some 99% of PSUs for PCs use Electrolytic Capacitors which age quickly, you need the spare capacity to avoid short life spans and to accomodate an extra drive or changes later.
And I'm going to upset some folk here. I see this issue with more AMD than other CPU based PCs. Don't know why but it's out there.