DDR2-667 Doesn't Hurt
The same issues apply for DDR2 that did for the DDR and regular SDRAM modules before them: there is no way the memory can work as fast as the theoretical ratings. In other words, we know that a memory module will always run slower than the ratings indicate. Also, it is possible to use faster memory than a system calls for: DDR400 DIMMs can be operated as DDR333 in a Sempron system, just as it's possible to put DDR2-667 modules in a computer that only supports speed ratios for DDR2-533.
So, even though none of today's chipsets supports the faster DDR2-DIMMs yet, there are nonetheless several good reasons to buy DDR2-667 modules right now. For one thing, you can generally select improved timings at just 266 MHz. Plus, the fast memory may be usable on future systems, where the memory required today might not. For another thing, the higher maximum operating speed enables simultaneous mega-overclocking of the system by increasing the FSB speed.
One common scenario practiced by enthusiasts is to buy a 915 or 925 motherboard featuring DDR2-667 and a slow-timed Pentium 4, and subsequently overclock the system to the CPU's limits by raising the underlying clock speed. Ideally, the FSB speed should then be 250 MHz rather than 200 MHz, which in turn enables a jump in the DDR2 speed from 266 MHz to 333 MHz. Naturally, the result is significantly enhanced overall performance.
Provided you have a suitable CPU, some boards can even be overclocked to just shy of the 300 MHz area. Abit and Asus are the two companies with the biggest reserves in this respect. Do note, however, that the effort required to cool such systems is not inconsiderable, and so we can by no means recommend such solutions for the vast majority of users. But given a system speed below 266 MHz (FSB1066) and some decent hardware, they can be set up to run in a stable manner.