I am pretty sure this has been covered here all ready, but you did say that you did not know of any specific setting in the BIOS for disabling the IGP.
This tells how to do that...
However, from it looks like they're saying, you already did that based on your previous statement of having it set to "PCI-E/PCI"...
Do you have any other Video Cards you can try; you never did tell us what the test result was with your 6850?
Also, and this could be a shot in the dark for me as I am not sure if this is required for your Asus Board... but, seeing as you're only using 1 GPU; then are you using a Riser Card in the opposite PCI-E slot?
I know some Asus boards require a Riser Card in one slot, if you're only using 1 GPU; otherwise the GPU won't work and you won't get the full x16 on the GPU. On my Sig Rig Asus board, I have to have my 6950 in the second PCI-e slot, with a Riser Card in the first (top) PCI-E slot for it to work; if not then I get the exact same problem you get. System will post, but no video from GPU; while IGP works fine... but I put in the Riser Card into the first slot, and the GPU then works fine.
The Riser Card was included with my Mobo.