I'm hereby letting you know that I gave up on this.

Why?
I tried the harddisk method today and I actually suceeded to boot like this. But when Windows was booted up (or rather woke up from hibernation) it said something about that the video drivers crashed and were restarted. I found that kinda odd.
Meanwhile the PSU was pretty loud (just like the ancient harddisk) and made noises I found worrying.
But as I got this far, I thought it couldn't hurt to try playing a game. So I started MW3. Shortly after it loaded, the PSU went out, and of course so did the graphic card. I guess that means that that PSU was too weak to handle only one GTX 570.

So knowing that that PSU is too weak anyway, I guess I should stop trying this and just wait until the new/repaired PSU arrives.

Thanks EvilSOB for all that advice! I really appreciate it.