After a fair bit of research and reading of reviews, I've brought together the factors that I think are important to this decision.
First, a Word About Efficiency and Heat Output
All power supplies draw more power from the wall socket than they provide to your computer equipment. The extra power is wasted - mostly as heat - during the transformation from AC to DC and to the voltages that the various outputs require.
A decent power supply these days will be 80 Plus rated - meaning it will be 80% efficient at peak power draw. The best ones might be over 85% efficient. They will all be more efficient at 50-80% of peak. I want my PSU to be at peak efficiency while my system is at peak load, because peak efficiency means the least amount of power being converted to heat - thereby helping case cooling. If it can also be efficient at average load, even better.
Now - How Big?
High-end CPU's and GPU's require more power. Peak power draw on an overclocked CPU or GPU can easily hit 300W and more. Hard disks are in the range of 5-12W, when loaded. (SSD's seem to stay below 2W, so they shouldn't cause too many headaches.) A water cooling pump will be up to 25W. Case fans range from less than 1W, up to about 4W. Other items are typically not so big as to be a major consideration - unless you have a lot of them.
If I'm going to have enough juice for a CPU and 3 GPU's, there's around 1200W (peak draw) right there without even thinking about the rest of the system. Figuring in 2 SSD's, (4W,) 3 HDD's, (25W,) 5 case fans (say, 10W) and a water pump, (20W,) leaves me at 1259W, with no headroom for anything extra at all. There are a few power supplies that will pump out that much juice and more - all the way up to 1600W - but they are awfully expensive and tend to run hot, noisy and inefficiently. It would be best if I could get away with one of the smaller units.
Fortunately, it is virtually impossible to have every component drawing peak power at the same time. Even in a perfectly balanced system, at least one of my 3 GPU's will be waiting on the CPU to send it some work to do, which in turn might be waiting on the hard disk to return from an I/O request. I am unlikely to be sucking data off of all of my hard disks simultaneously. (To be fair, the pump is likely to be maxed when the rest of the system is maxed, because that's when it will be generating the most heat.) The point is, I'm pretty unlikely to be drawing 1259W at a time.
I'm guessing that my GPU's won't be maxed out while I'm reading my hard disks - games generally avoid disk I/O during gameplay, as you don't want your expensive, lightning fast graphics card(s) waiting on a slow old disk operation. I'm guessing that a more realistic peak power draw for the system will be around 75-80% of the CPU and and GPU's combined peak, plus the pump and fans, plus the rest of the system at idle. That gives me a guesstimate of 1200 (CPU + 3xGPU's) * 0.8 + 17 (SSD's and HDD's at idle) + 30 (pump and fans) = 1007 watts, peak power draw.
Trying to fit my system into the peak efficiency band of most PSU's further increases this number. 1007W is 80% of 1259W - so I'm still gong to need a big power supply! They don't tend to make them that exact size, but they get pretty close. They seem to be available in 1000W, 1050W, 1200W, 1250W, 1500W and 1600W varieties.
The high-output PSU's tend to be less efficient, too. I couldn't find any 80 Plus Gold rated PSU's over 1000W - and precious few rated 80 Plus Silver.
Do Overclockers All Need Single-Rail PSU's?
This post from jonnyGURU seems to be the authoritative word on single rail vs. multi rail PSU's; I recommend it to anyone that is considering buying a high-end power supply. Basically, if you want any inbuilt protections against short circuit, (e.g. you do not want to get electrocuted or your house to burn down) get a multi-rail power supply. This will incur the overhead of having to be more careful about how you plug your system together, so that no single rail's capacity is exceeded - or your computer will shut down.
You will also need to be careful to buy a high quality PSU that has enough rails, with enough capacity, to power your equipment. If your highest capacity 12V rail is only rated at 20A, then you won't be able to drive a high-end CPU or GPU, as it will shut down if you try to draw over 240W. (12V x 20A.) If you're overclocking, you will likely need even more current on your rails. You may also find yourself in the situation where you are unable to draw the full rated power from you PSU because (e.g.) you have a component needing 30W and your 6 rails only have 10W of capacity left, each. This last is more likely to be a problem when you are near the limit of your PSU's capacity, anyway - another reason to allow yourself a little headroom.
Single rail PSU's do away with this configuration headache and allow you to push your PSU closer to its limit by providing its full capacity with one gigantic rail.
How big did you say?
The more mundane side of buying a high-end power supply is that a higher output rating will typically mean a physically larger unit. Some PSU's today are so big that they won't fit in some cases. Others will fit, but might prevent a 5.25" bay, or perhaps a hard drive enclosure, from being used. Whether this is a problem depends on your case and what you are trying to fit into it.
I'm planning on using the Lian-Li PC-P80, one of the few 10-slot cases on the market, giving me the option of 4 dual-slot graphics cards. This case will fit a maximum PSU length of 220mm - just short of the largest PSU's about.
PSU manufacturers - particularly high end ones - are starting to pay more attention to noise reduction now. Units with bigger fans, moving more air at lower revolutions, tend to stay cool for less noise output. Some of the less powerful units even get away without cooling fans. I want mine to be as quiet as possible, but higher end units are going to have some fan noise.
So, which one?
I looked at the reviews for a number of units, before settling on the Enermax Revolution 85+ 1250W. It is 190mm - leaving 30mm more space than the Silverstone Strider ST1500. It gets a "Best in Class" award from Hardware Heaven. JonnyGURU has good things to say about its cousins. It is over 89% efficient between 500W and 1125W power draw, which should be my normal power draw. If/when I eventually hit 4-way SLI and max this unit out, I can always add one, or even two, of these in 5.25" bays to take care of the excess.