Seem to have spent much of the past week out somewhere past the bleeding edge...
Put together an Athlon64 box with 4GB memory a couple years back. The box ran Linux, served as a Samba file server to the Windows boxes in the house, and hosted test machine configurations running under VMware. After several months of use, realized the memory was not running at the rated speed, and wasted far too much time fiddling with settings (with some help from the manufacturer) - without any improvement.
Since then the price for the Athlon64 X2 (dual processor) CPUs have dropped to the impulse-buy range. Aside from the general performance improvement, there are cases where testing against a dual-CPU box would be helpful. So ordered an Athlon64 X2 4200+ and a new CPU cooler (I had hopes of overclocking). Dropped the new bits into the box, updated the motherboard BIOS to the latest, and spent a thoroughly unpleasant week afterwards.
Tried overclocking the CPU slightly - no joy (errors from Memtest). Dropped the CPU clock to stock - no problem. Tried raising the memory speed from DDR333 up slightly closer to the rated DDR400 - no joy. Even more confusing, I got occasional errors running tests at DDR333 - which I eventually realized only occurred immediately after testing at a higher speed. That was the clue! When running tests above DDR333 found the memory was blazing hot to the touch! Immediately turned the CPU cooler so that it now pulls cool air over the memory (rather than blowing slightly warm air as before). Ordered a memory cooler - but it does not fit in my setup. Looked closely at the memory (from OCZ) and realized the fancy metal "heat spreader" is too thick! - no air can get in between the sticks of memory. For those folks with only one or two sticks, this might not be a problem, but with four sticks the entire set got very hot.
(Something almost funny about fancy-looking "heat spreaders" that cause your memory to overheat. Almost not-really-at-all funny.)
Last time around I had meant to setup the box with a 64-bit version of Linux, but somehow ended up installing 32-bit. This time meant to have a full 64-bit setup, so installed the amd64 version of Ubuntu 7.04. The installation was no problem, but then I made the mistake of getting a bit ambitious. Only 3GB of memory is reported as available of 4GB installed. Saw an error/warning in the Linux startup log, and chased down a reference for how to make that last gigabyte usable (Ctrl-F1 when in the Award BIOS setup, and ...). This looked great - 4GB usable memory! - except for the fact the network interface stopped working. No joy figuring out why. After much fiddling around (and re-installing Ubuntu, again) reverted to the original settings.
Linux is suffering from an odd sort of fragmentation and perhaps a bit too much popularity with unskilled users. Trying to find hard information involves shifting through a lot of junk. Finding someone who might know more ... I gave up after a while. I would be perfectly happy to help another developer track down the problem, but I gave up on finding the right starting point.