I know what voltages the cables from the atx have. The only thing i dont know is if i can safely take a 5V and a GND cable from the atx and power the rpi through its pins with it. I dont want to break it and for sure im not the only one coming up with that idea. Unfortunately i only found people that created a usb-connecter to power the rpi with a psu.
Then check this out:
http://potech.dyndns.org/Easy%20PSU%20for%20RPi.htmlThis confirms my plan, thanks

That's what I'll do once they arrive

Be sure to use one of the +5V lines and not the +5V standby if you want the pi to go off when you turn off the power supply. Also, be sure to check the rated current output of the standby lines, it may be lower than the full +5V rail. You need 1A for the pi to be happy.
-Jay