Ok, there's some misinformation in this thread
First off, HD's only draw between 4 to 5 watts. Raptors and SCSI drives might draw a bit more, but it's not even 10w. Moving to an SSD will save only a few watts and is completely ridiculous in this application. If you're going to do that, just make a bootable USB key and boot into Linux that way.
Second people aren't seeing the issue because most boards that designed for 2x SLI, 3x SLI, 4x SLI or Cross/tri/quadfire area designed with the idea in mind that current draw on the board is going to be substantial, so they are beefed up. Usually, (but not always), anyone running such a beefy system will also have a beefy power supply with large gauge wires to handle the current draw. In the cases we are seeing here, people are using commodity boards and cheap PSUs, which are explicitly NOT designed to handle these kinds of extreme loads... but since this is a fringe area of computing, the problem doesn't crop up that often.
Most low and mid-level boards are going to sag power wise with even 3 cards in the slots. You can get away with probably 5870's, but if you add a 5970 or two to the board and expect it to run, you're going to have a surprise or flaky operation. I know this from experience. Using a high quality gamer board will alleviate this, but has a cost involved. To solve this, splicing off and using MOLEX reduces or eliminates the draw on the board. One of the problems I have seen, or at least this is my speculation, is that even if the board works for awhile with 3 5970's in it drawing off the MB, the VRMs will start to heat up and eventually fail if that much current is being drawn continuously. Again, taking that power draw off the MB and putting it on the PSU, which is designed to handle far more load than 75W @ 4A+ is going to eliminate those problems, extend the life of your board and be far more stable.
The problem I'm trying to work through now is whether or not a common ground is needed and if so, the best way to handle grounding two PSU's to a common ground if you wanted to use two cheaper PSUs to power a mining rig.