Your final statement is the key point. Electrically, those adapters are fine, but if you need to use them, odds are high your PSU isn't up to powering whatever you want to attach. If you have something like a 380W Antec Earthwatts, you could run a GPU like a GTX460 or HD6790 without problems, but if you've got an older unit (e.g. anything with a little voltage switch on it), you probably don't want to take the risk.
I bought the wrong power supply - no fault but my own it was stupidity and not thinking. I had a power supply, it died and I made a choice in 5 minutes in store without doing any research.
The power supply I have is a Corsair TX 650 - it has 52 A on a single 12v rail. There is lots of power to be had and it's sufficient power for what I want to do, which is crossfire my AMD 5870.