I'm gonna try and help you out as much as possible nacho, I have the same exact card with no problems and i hate seeing that you have been fighting this thing for so long.
Ok, Lets say the card is not DOA, i have a feeling that your power supply or they way you have your computer setup is the reason its not working.
I got my card in yesterday, tested it today in a Nforce comp, Specs:
Nforce mobo.
Core 2 Quad 2.4
3gig RAM
2x500gb HD
Roswill 600Watt PSU
HP video capture card
This is not my system, its my friends, i just use it to make sure my card wasn't DOA, i'm missing the OS and PSU for my system.
We had minor problems and it was power consumption, the box states that a minimum of 550 Watts is required, that is wrong!
Even the Roswill 600 Watt had trouble powering this beast, it was stealing power from the hard drives and wouldn't boot. We had to remove the slave drive to have enough power to boot. After that it was like dropping a V8 engine in a volkswagon beatle!
I'm going to say a minimum for this card to work is 650-700 Watts.
Now i can only think of a few things with your setup:
1. make sure you have 2 6-pin plugs connected to the card, it really needs both of them. and don't use the adapters that come with the card, use the ones on your PSU, if it doesn't have it then thats a problem.
2. Your setup is just like my test, try removing any non-needed drives or PCI cards. just try booting with the bare minimum.
3. Honestly i think this is the problem, That PSU your using is not SLI or Crossfire certified, now i know your not using a crossfire system, but this card is the idea of a crossfire merge, its basically two 512mb single core GPU cards merged into one, even if you crossfire 2 of these things it actually see's it as CrossfireX (quad crossfire). if everything else fails, try a SLI/Xfire cert PSU.
4. Highly doubtful but i'm just saying this, maybe the mobo is bad, if you have a cheap, working card around test it with that and see if it boots.
I hope you get this card working, i know you will like it.