When choosing memory, you need to consider three factors:
Speed (DDR2-553, DDR2-667 or DDR2-800)
In Core 2 Duo systems, memory is connected to the CPU via the front side bus (and the memory bus) whose maximal theoretical bandwidth is
1066MT/s x 64bit/8bit = 8.533GB/s.
(Pentium 4 and Core 2's FSB is 64-bit wide.) That only equals the bandwidth of dual-channel DDR2-533 or PC2-4200:
533MT/s x 64bit/8bit x 2 (dual channel) = 8.533GB/s.
(DIMMs have 64-bit data paths.) Therefore there is no big performance growth if you use memory faster than DDR2-533. This is confirmed by various benchmark tests in the AnandTech and X-bit labs articles. The story for Athlon 64 systems is slightly different. Because of the superior on-processor memory controller, AM2 has a wider memory bandwidth and its performance depends more on memory speed than Core 2 Duo. But playing movies is insensitive to memory subsystem and there won't be much performance difference even if the memory speed changes.
