This is only recommended if you haven't got enough power to your system and you're running a few fans and hardrives cddvdroms
WARNING: read the comments for safety measures
You will NEED to link the ground connections in order to make this a lot safer. When I wrote this I was unaware of the voltage on data lines (Sata, IDE ect).
All you need is
A second powersupply (wattage dependant on what you are running but most likely even a 300 watt one will do.)
A paperclip or a small cable
And somewhere to place the second PSU (inside the case is recommended but not always advisable depending on the amount of room you have inside your case.)
You may also possibly need electrical tape (optional to cover the paperclip and prevent short circuits) and power extensions (IDE /SATA) if the cables are unable to reach to your devices.
A WARNING DO NOT plug in the 24pin or the 12pin creating a mix of power between the first and second PSUs is not a good idea as it may damage your motherboard.
Plugging only one PSU into the motherboard is appropriate.
Its advised that you only do so with the case PSU otherwise you may not have power to your hardrive (unless you plug that into the second one too) when you turn it on (not dangerous just useless).
Step 1: Jump Start.
The first step is to put in a paperclipwire into the GREEN pin and into a BLACK pin (On the 24pin/20pinconnection). This can be different for different units. But the colours are always the same. This is the on signal (green) and ground so when power is going to the power supply and it is switched on at the unit it will jump start.
You may want to tape up the connection after doing this. Just to be safe and not cause any short circuits.
Step 2: Connecting Up Peripherals\fans.
Now you need to connect up all the hardrives\cdroms\fans etc.
This is where an extension would come in handy if you have it outside of the case.
Step 3: Turn It on and You're Done.
Turn it on and tah dah. Your Videocard (and cpu if its a single 12V rail main PSU) will now have enough leeway to consume a lot more power (depending on how many peripherals were connected.).
Don't forget to turn it on before you turn on the computer though. (useless if you don't because you hardrive won't turn on.)
Enjoy all that extra power ;).