We consumers never learn this lesson well, no matter how many times we get bitten.
Always read the fine print.
I bought an additional pair of DDR-400 memory modules for an old gaming PC a couple of days back. It was not state of the art technology when I bought it in May 2006, but I did not need much to play World of Warcraft. By maxing it up to 4x 512MB I now have a comfortable 2GB of RAM for most of my gaming needs. I thought it would be that simple.
On boot up I immediately noticed the BIOS reporting 2GB cycling at 333MHz. If I operate with only a pair of modules, the RAM rightfully runs its advertised 400MHz double-data rate. Plug in all four, it dumbs down the frequency (read: speed).
I consulted the motherboard manual and to my horror, read some very blatant tabular facts that if all four slots are fitted with double-sided modules, the end frequency clocks down to 333MHz. Thankfully, within the hour I was able to locate some suggestions on how to resolve the issue - manually set explicit settings in the BIOS instead of letting it decide in Auto. Going by past explanations in other places, each side of a memory module is considered a bank. So even though one slot is taken, a double-sided stick constitutes two banks. Apparently, AMD-based motherboards have for a very long time sported some limitation in the number of banks (threshold 6) it can handle at higher speeds. Exceed the threshold, and it is deemed unsafe and unstable as it struggles to synchornize the signals between all the memory banks. Testing my PC at 400MHz with 3DMark06 and Company of Heroes however, did not reveal any weakness, much to my relief.
In all my years of using DDR-266 RAM I have never encountered or read about such issues, so this totally broke my concept of "RAM speeds as advertised". Ouch. I was caught off guard. How much more for the layman user?
UPDATE 4 Jan 08: for a game that is more computationally and memory intensive, I would have expected Company of Heroes to bomb. It did not, but World of Warcraft occasionally does. And it crashes with an irritating screeching sound. Ouch. Having defaulted it back to 333MHz I have not experienced any crash ever since. Damn.