I think the thing you want to worry about is the amp draw on your 12v rails on the motherboard. These are going over the solder traces and there is probably an amperage limit that you can hit before things get too hot and turn into carbon.
The three most basic units in electricity are voltage (V), current (I, uppercase "i") and resistance (r). Voltage is measured in volts, current is measured in amps and resistance is measured in ohms.
The voltage is equivalent to the water pressure, the current is equivalent to the flow rate, and the resistance is like the pipe size.
Let's see how this relation applies to the plumbing system. Let's say you have a tank of pressurized water connected to a hose that you are using to water the garden.
What happens if you increase the pressure in the tank? You probably can guess that this makes more water come out of the hose. The same is true of an electrical system: Increasing the voltage will make more current flow.
Let's say you increase the diameter of the hose and all of the fittings to the tank. You probably guessed that this also makes more water come out of the hose. This is like decreasing the resistance in an electrical system, which increases the current flow.
Source:
http://science.howstuffworks.com/environmental/energy/question501.htmSo the powered risers essentially bypass the thin hose on the motherboard and supply the voltage right at the card. It's a good theory and decent practice when applied properly. I wouldn't power all your risers, maybe a couple of them. Better yet, find out the amp rating on your rail(s) on the motherboard and then determine the amp draw from your video card and compare the two.
