HELP!! 790i Ultra SLI Wont Boot with 8GB of RAM!

I have a 790i Ultra SLI Mobo from EVGA. I bought 8GB of DDR3 from OCZ's Reaper line. I have all the settings correct based on their 7-7-7 1.9v profile. Windows vista 64-bit will not load and give an error message that requires the system disk. Yet the windows system disk crashes to blue screen. If i run only 4GB vista boots fine and without a hitch. I have tried retarding the latencies and frequency and then windows starts to load but froze on the windows is loading screen. I am so frustrated! What do I do. The mobo says it takes 8Gb, but ti clearly doesn't. HELP!

MANUALLY set your voltages to the upper range for your RAM. MANUALLY set the timings. If you have a voltage drop control - enable it. Ensure the command rate is set to 2T, and that tRFC ("Refresh to activate delay" ) is 54 or greater.

If that doesn't work, you may need to raise the voltage to your FSB A Couple Steps, but no more. If you have to crank this up, go back and see what else could be at fault.

I agree with scooteq (though I'm not sure I understand the last statement about looking for what else is at fault if you have to up the MCH/Northbridge voltage).

To that point, on a different thread there was a good explanation of the potential issue that you may be facing. I'll include it here (I had saved this away so I don't remember who first posted it, otherwise I'd give them the credit due...).

You've run into a common issue (not mentioned above). Having 4 DIMMs installed at once is pushing the limits of current unbuffered DDR2 RAM technology. The control signals to the DIMMs get split among all of them, so having 4 installed means the signals to each are much weaker than with only 1 installed, causing errors. The better the motherboard design and the better the quality of the RAM, the less likely the problem, but once you have it, the only solution is to try raising the MCH (memory controller/northbridge) voltage a bit, perhaps 0.05V or 0.1V. This is NOT the DIMM RAM voltage (which you should have already set to the spec voltage specified by the RAM manufacturer for your specific DIMM part number).