I am using the Asus version of this board, the M2A-VM. It had the white line problem when using over 4GB of RAM on Vista 64 bit. Like the other boards mentioned here, it had the AMD 690 chipset. I believe it was the RS690G, but I don't think the details are important.
Tonight, I upgraded the BIOS to a brand new one that was just released. It seems to have solved the problem, finally. I now have almost 6GB available in Vista and no white lines.
If this keeps working, it is clear that the problem had absolutly NOTHING to do with drivers, memory, Vista, hard drives, msconfig, or Windows patches. That is evident by the fact that it now works and all I changed since the last failure was the BIOS.
This machine is now running 4GB of Corsair plus 2GB of OCZ memory for a total of 6GB. With under 4GB of RAM, it worked fine in Vista 64 bit. With Windows XP x64, it worked fine with the 6GB. I memtested each memory configuration and all were stable so I knew it was not a memory issue. I did have to load Vista with only 2GB and then put in a Windows patch when I originally loaded Vista. Vista is loaded on a SATA hard drive, but not RAID.
I am running defaults on all the BIOS parameters except memory voltage and almost all Vista parameters. The settings were not part of the problem. The memory voltage had to be set up because this board does not have enough power to run all the memory. Before setting it higher, I could not get a stable memtest.
Before tonight, I had the most current of everything. All my hardware components had been checked several times, for hours. I spent a lot of hours on this. The Asus phone support people assured me that they had never heard of this problem and therefore it COULD NOT EXIST. More recently, I did talk to some higher level tech support people who did acknowledge the problem, but the level one people denied it existed.
This has been going on for months. Lets hope it is finally fixed. For those with other brands of board, I hope your manufacturer has seen fit to release their version of this fix. The Asus one did NOT say it was to fix this problem, hopefully your manufacturer will be more honest about it.