Thanks for your advices.. I tried with my main computer, replacing the nvidia gtx 470 by the 5870... No problem, it worked, I saw the bios settings and I was able to boot windows 7...

The problems seems to be related to the motherboard despite I though, like BinaryMage, that was backwards compatible accordingly to wikipedia...

Next step is probably to get a new motherboard, that mean to build a rig for bitcoin... I'm not sure it's very interesting to do since the quick increase of difficulty?

So unlucky !

If you see something other that I can try please tell me