I've got some big wattage server power supplies with plenty of 12v rails and I was wondering, if I had a normal power supply to run the CPU/Motherboard, could I run the external power for the video card (8-pin) off a separate power supply? Not saying there would be a lot of reason to do this, just wondering if it's reasonably possible/practical. The concept of a dedicated power supply for a video card just seems too bizarre not to ask.
It is possible to do but is undesirable. There were units dedicated to doing this a couple of years back and a quick check shows you can still get them.
I believe from memory the main Con is wanting matching voltages but i may be wrong.
Hopefully this link will help explain it http://www.hardocp.com/article/2011/01/24/epower_juice_...
You can absolutely do this--you don't even have to be hesitant about it. Many extreme users link PSUs to power their TECs or GPU array.
They make cables for it now, you just need to link gnd with the green (pwr on signal) and purple (standby) cables and it will start right up with your main PSU.
Honestly though, you can be much lazier if you want. I have two such systems right now (on a box on my desk xD) powered by two PSUs each for testing our company's components. How do I have them linked? I turn on the system...then jump the PSU powering our hardware...
Most expansion cards/hdds don't need power instantaneously with the motherboard.