|"You don't usually convert to amps when talking about video cards. You talk in watts"|
That's fine, but you need a basic understanding of where that wattage is coming from. A PCIe x16 slot is capable of supplying up to 75W. Of that, 5.5A comes for the +12v, the remainder from the +3.3v. If you look at a 6-pin PCIe power plug, it has only yellow & black wires. Black, of course, is ground, yellow is +12v. And since this plug is also capable of supplying up to 75W, do the math: 75W/12v = 6.25A.
So if you have a PCIe x16 card that requires the 6-pin aux power plug, it can potentially draw up to 11.75A (5.5A + 6.25A) from the +12v rail, or 141W.
An 8-pin PCIe plug is capable of supplying up to 150W, all of which comes from the +12v rail. So 150W/12v = 12.5A, plus the 5.5A for the slot = 18A. Whether or not the card requires that much depends on the card, but when choosing a power supply, you need to take into account the potential of 18A of +12v being needed by the video card alone.
Modern CPUs are powered entirely by the +12v. Like the video card, the power requirement varies depending on the CPU. On the higher end, there are CPUs that require 140W, so once again, 140W/12v = 11.67A.
So with a high end video card & a high end CPU, you would need at least 23.42A (11.75A + 11.67A) on the +12v. But that doesn't take into account HDDs, optical drives, fans, etc. If you figure 2A per drive & 0.25A per fan, that would add at least another 5A to the total. Now we're sitting at 28.42A. So when Skip says 30A on the +12v is about the bare minimum, he's right in the ballpark.
30A x 12v = 360W, and when you add on the +3.3v & +5v rails, you should see why 400W should be the minimum size PSU used these days.
Now I realize that these numbers assume max load, but even so, do you think it's wise to potentially load 28.43A on a +12v rail with a max of 30A? That's cutting it pretty close. It's always smarter to get a PSU with higher wattage than is generally required...that way you have a bit of a safety buffer. But when choosing a PSU, you can't go by wattage alone. You also have to pay close attention to the amperage rating on the +12v to make sure you azz is covered.