DDR2-533 (266 MHz base clock) is the speed at which DDR2 was introduced in 2004, roughly in sync with Intel's 915 and 925 platform launch. More and more systems are based on fast DDR2-800 memory today, but there are still lots of systems that can only handle slower speeds, especially notebooks, which often still utilize DDR2-533 or DDR2-667 memory.
Most of the DDR2-533 memory that we found on the web is specified for 4-4-4 timings or faster. There is hardly any cost difference between DDR2-533 and DDR2-677, and hardly a difference in power requirements either: 1 to 3 W of additional power draw for DDR2-667 over DDR2-533. Accordingly, we recommend going for the faster DIMMs if possible.