Well right now I already have the main PC assembled with the 7800GT and so far everything is perfect, it seems that the problem is only with the graphics card. When I have some more time I will try the 6870 with the other PC, just to make sure it works.
At first glance the graphics card doesn't seem to be broken, although it's also true that the S1 with the fan doesn't let me see very well, when I remove it I will confirm something. Anyway, even if it blows up, there doesn't have to be anything physically broken.
I have a craving for nVidia, so I think a GTX560Ti is going to fall. And if I don't have problems with the RMA (I don't have an invoice, but the guy who sold it to me is a reviewer and bought it directly from the supplier, when I bought it I talked to him and in principle there shouldn't be a problem) I will sell the 6870 they send me 
Salu2