I'll have to warn you that this is a very sticky section and you should only select memory modules based on this metric if you do your homework. You will need to trawl forums and websites for memory chips that are particularly overclockable. There are literally tons of memory chips out there, like Winbond's famous BH-5, CH-5 and CH-6 chips to the Micron DBG9 and the Hynix BT and DT-D43 or BT-Js chips.
These chips are not only famous, they form the basis of most comparisons, so it pays to be aware of them and their characteristics. This is because different memory chips have different characteristics. Some memory chips can achieve astoundingly high clock speeds, while others are capable of ultra-low latencies. Depending on what you are looking for (high clock speeds or low latencies), choose your memory modules based on the memory chips best suited for the job.
While you can count on memory modules that follow JEDEC standards to work properly with all motherboards, this degree of compatibility is not possible for high-end memory modules. Because they are designed to exceed JEDEC standards by running at very high clock speeds or ultra-low latencies, they may not work properly with certain motherboards. For example, high-end memory modules from OCZ Technology have a notorious working relationship with some BIOS revisions on DFI motherboards.
Therefore, it pays to go through both motherboard and memory manufacturers' compatibility list. Obviously, the more motherboards a memory module has been successfully tested with, the more likely it is to work with motherboards that it had not been tested on yet. But if your motherboard-memory combination is on the compatibility list, then you know they will work for sure. No guesswork, no head scratching.
Athlon 64 Memory Controller Limitations
The AMD Athlon 64 processors are quite special in a sense. Unlike traditional processors, it actually has an integrated memory controller. As you know, the memory controller is traditionally part of the north bridge. AMD chose to integrate the memory controller to improve its performance. The tighter integration reduces latency and improves memory thoroughput to the processor.
However, the integration of the memory controller has some drawbacks. Timing requirements are tighter and there is a limitation on the memory load. For example, the older Socket 754 Athlon 64 (Winchester core) processors could only support two double-sided modules at 200 MHz (400 MHz DDR). You can add additional modules, but that would instantly force the memory controller to drop the memory clock speed to just 166 MHz (333 MHz DDR).
The Socket 939 Athlon 64 processors also suffered from some of these drawbacks, but to a lesser extent. They would only drop to a lower speed when more than three double-sided modules were installed. The newer Venice cores of the Athlon 64 didn't suffer as much and maintained its speed of 400 MHz, but installing four double-sided modules would drop its command rate to 2T.
Of course, AMD is continuously improving the characteristics of its integrated memory controllers. We will see the latest iterations in the upcoming Quad-Core Opteron (Barcelona core) and Phenom processors. Be sure to keep your eyes out on reports of limitations of these memory controllers, as well as those in the new Socket AM2 Athlon X2 processors.