If the BIOS shows 4 GB, you can't expect Windows to see 8 GB. I checked the QVL and I didn't find any certified 4 GB modules. Does Corsair have your motherboard in the list of compatible motherboards? If so, I couldn't find it on their site.
Edit: What happens if you install a single module?
Use BIOS Version 0904 or 1001. Often the P55's (LGA 1156) have had 'bent CPU pin' problems causing this problem. Install the RAM in the 'Blue DIMM' slots.
First, in the BIOS check 'Memory Remap Feature -> Enabled' ; required for 64-bit OS and >4GB RAM.
Q1 - What changes have you made to the BIOS?
Q2 - What exact CPU?
Q3 - What exact set of 2x4GB Corsair Vengeance?
Q4 - Other than the 1 beep any other beeps during booting?
Q5 - Any Partitions with 32-bit? though the 3.99GB indicates a BIOS turned-off DIMM/Stick.
You can try the 'Black DIMM' slots to rule-out a bad DIMM slot.