I ordered my 2gigs of RAM on Thursday from NewEgg and had it overnighted. When I got the package, I excitedly ripped upon the box, installed the 4 512MB modules (and an HDD for Longhorn testing), and hurriedly turned on the 'puter. I looked for the hardware info to make sure the system was registering the 2gigs of RAM at 200Mhz. The 'puter recognized the RAM, but not at 200Mhz (instead it was running at 166Mhz). I rechecked the package: sure enough, PC3200 RAM. I ran CPU-Z and it said each module was PC3200, too. So I went searching.

I found that the memory controller on Winchester core CPUs (AMD64) supports single-sided RAM, and only partly supports double-sided. When all four RAM slots are taken with double-sided modules (or slots 1 and 3 to be more precise), the max bus speed is 166Mhz. Supposedly the new core (Venice) will have an updated memory controller that offers full support for double-sided DIMMs (as well as SSE3).

I want the 2gigs, so now I'm pretty much stuck at 166Mhz until I upgrade the CPU (which will be a while). Cheers to AMD for not thinking people would want more than 3x 512MB modules in their system.