Upgraded To 4GB - Windows 7 Boots And Starts Up But Getting Blank Screen

Windows boots and starts up, but at some point I get a black screen from which I cannot exit unless I reboot. Since my video card has 512MB, and my board's maximum memory is 4GB, I suspected Windows would not have the full 4GB to play with, and, sure enough, the Windows properties screen shows I have 3584 MB available. Also, the POST screen seems to test 3670016 KB, which I believe is the same thing. In fact, even the memory test programs report only 3584 MB memory. I have been running Win 7 x64 for several months on my Gigabyte GA-K8NSC-939 motherboard using 2GB memory (4x512MB). I decided to upgrade to 4GB (4x1GB), the max the board can handle.

Installed and tested using Memtest-86, Memtest86+, and Memscope with several passes of each -- all good. On the other hand, one of the advanced tests for Win Mem Diagnostic failed, but since it was making lots of artifacts on the screen, I suspected it was confusing video memory with RAM. So I thought Windows knows about my memory and vid card and is reserving things apprpriately, but maybe not. I boot and go into Win 7, and enter my password, but some time later the screen goes dark as if it has gone to sleep, except it will not wake up.

Most of the time just before it goes dark, I see several colors on the screen, which make me think it has something to do with the video, and I wonder if the computer is getting confused about which memory it can use for what purpose. I looked in the BIOS for something obvious, but I didn't find anything, except (maybe?) the setting for "Init Display First" which was set to "PCI slot," and which I changed to "AGP," but I don't think that worked. I looked for but did not find something about memory mapping, so maybe that is on already.