I know that I can use any PCI-e slots... if I've the risers.
I've to order some risers but until then I have to fend...
Now, I've two 7950 that are put in the PCI-e x16 slots... but one of them has a high temperature (they are too closed).
Then, if I put one 7950 in the x16@4x black slot, They are farther
The x16@4x black is the last slot
This is my mining rig
http://imageshack.com/a/img585/2840/xlgg.jpg