Solved 64-bit Win7 Can't Handle 4 GB RAM Without BSOD/reboots?

I've put together a new clone system with the intent of running 64-bit Win 7 Pro on it. Until it becomes available, I have 32-bit XP Pro installed on it. Nothing is overclocked, in fact, the memory is "underclocked" for stability. I'd rather have it stable than gain some trivial performance improvement.

Previously, when I first built it up, I installed 64-bit Ubuntu "Jaunty" on it with 8 GB of RAM installed in it (4 X 2GB). The Linux distro installed and was perfectly stable.

To get a "heads up" on Win 7 issues, I downloaded and installed the trial Enterprise edition. Since I couldn't find any MD5 for the ISO, I downloaded the file twice and did a binary compare on the two downloads to ensure they were the same. Also, I see that the Java applet that downloads the ISO file also has some kind of "verify process" built-in that runs at the end of the download (a checksum?). I burned a DVD in XP Pro using Nero 8 with data verification on (after burning, it reads the burned disk and compares the disk's contents with the ISO file).

Since 32-bit XP Pro can't even fully use 4 GB, I uninstalled 4 of the 8 GB (two modules) when I installed XP on the box. When the entire 8 GB was in the MoBo, I ran MemTest86+ for long periods without errors. To do so, however, I had to "underclock" the memory to DDR2-800 speeds. It couldn't successfully run at DDR2-1066 speeds. A problem that, by checking various support BBS's, was found to be common with my MoBo and the RAM modules in question. After pulling the two modules, I left the memory timing at the conservative settings. I also let MemTest86+ run overnight and got no errors with the two modules installed. My plan is (or, was) to put the entire 8 GB of RAM back after I got 64-bit Windows 7 running on it.