No display / 1 long 2 short beeping error

Hello, I've been having a slightly odd problem with my pc, I bought it yesterday and it was working fine, but the GPU (Nvidia 620 GT) really bottlenecked my gameplay considering it's quite a bad card, so i wanted to replace it.

I went and replaced both PSU and GPU from my older pc into the new one, but it wont boot. I'm certain the PSU or GPU isn't damaged considering i used it a few days ago and it was working fine (nvidia 580GTX with 550w powersupply, current CPU is i7 3770)
Nevertheless I don't get any video on startup and i get 1 long 2 short beeps (indicating video is faulty) eventhough it isn't. So i changed the GPU's again and inserted the new one i got together with the system, but that didn't work anymore either for some strange reason. I got the same issue with the beeping and no video. I resetted CMOS and reseated the card, this apparently worked since it would start up again like normal, so i thought i might have to reset CMOS with the other card aswell, didn't do much since i still got the same error.

At this point i thought the GPU was probably faulty, so i tried another older card i had laying around (ATI 5750HD) but that one didn't work either, got same issue as before, no video and 1 long 2 short beeps.

The only thing different with these cards is that the ATI and the GTX both require those 6pin cables (no idea how to call them in english) to power the GPU

Could my PSU be faulty? eventhough it worked fine on my other system just a few days ago.

Any insight or help on this would be really helpful. (P.S.: I did the checklist on booting)