|With the one module in, try resetting BIOS to defaults, Save and Exit. Then going back in and manually making sure that the CPU, RAM, Time/Date, and graphics are set correctly in the BIOS. Adjust as needed. Save and Exit.|
One more thing, I just want to confirm that the memory you purchased is a matched set and came factory sealed in one single package. Individual memory sticks can be of different settings and if far enough off from each other, cannot run together, cannot run on a single channel, or cause other problems.
You might have a bad stick of memory. With one stick only, boot to a CD (bootable) with Memtest86 on it and let it run through all tests. No errors are acceptable. Shut down and repeat with the other stick. If one has errors, send them both back for replacement under warranty. I believe I remember that Memtest reports details on the memory, write these down and if the specs differ significantly, get them replaced as well. If the package was not sealed, they may have been a returned package for the same problem and was resold to you instead of being marked defective.
You have to be a little bit crazy to keep you from going insane.