In a hypothetical situation where I would like to replace my graphics card with a brand new one that would increase the power required from the PSU to over its capacity, is there anything wrong with using an old, separate PSU just for the graphics card?
While you can possibly reuse an old PSU depending on it having the connections at the proper voltages that your new graphics card is looking for, you can great a great PSU for under 150 bucks. If you're spending all the money for a new graphics card, you really ought to spend a little extra for a decent PSU that's sure to have what you need, be more reliable in the long run, and doesn't require you to "frankenstein" it up as Shinrai put it.
I will assume that your secondary power supply has 1 or 2 PCI-E connectors (depending on what the graphics card needs. I assume that this power supply is 300 Watts or more.
Find a place to permanently mount / affix the power supply - this may involve duct-tape, tinsnips and band-aids (DO THIS AT YOUR OWN RISK) * Make sure the PSU is not plugged in to a wall outlet at this time.
Connect the 1 or 2 PCI-E power connectors to your graphics card.
Locate the 20 to 24 pin ATX connector (depends ont he power supply, it's the largest group of wires / connector that normally plugs into the motherboard).
Once you have identified the ATX connector, look at the separate wires, you will notice there is only one green wire. Take a moment and observe that there are several black wires, you can choose any one of them for the process, it is best to choose the closest one to the green wire.
You will need a short piece of wire that is stripped of its coating at both ends. You will now insert one end of this wire in the pin that corresponds with the green wire. You will then insert the other end of your wire into a pin that corresponds with the black wire you identified in step 4.
At this time, you now have a PSU that will power your graphics card. You can plug it in and flip the switch on the back of the PSU.
You will probably want the graphics card PSU on before you turn the PC on itself or it may not recognize the graphics card... if things don't work out... you may lose the graphics card, the motherboard on the PC or BOTH...
Use a original style xbox360 power supply, the wires are all the same as pc. It will only give you 200W though, so it won't work on some high wattage cards. Make sure it's not the slim version xbox, that one is only 120W.
Just cut the end of the power input off and strip the 3 black wires and 3 yellow wires. Get a 6 pin pci-e connector and splice wires to corresponding colors.
You will be left with a blue wire. That's the sensor wire to turn on and off the supply when it gets a 5V signal. Get the 5V from either the last red wire left over or if you want to be fancy, to turn it on you can have the blue go straight to any 5v red wire output somewhere from your pc power supply or from a molex connector then they both turn on and off at the same time. If u did it the first way where you short red to blue, you could put a small switch in between if you wanted, or just unplug it every time the pc is off.