PC2-6400 is capable of running on a faster bus (400 MHz, 800 effective) than PC2-5300 (333 MHz, 667 effective).

 

Assuming you have a notebook computer, no current notebook using an Intel chip(set) can run RAM at 400/800 MHz - there's no point in buying PC-2 6400 RAM.

 

 

Currently the only reason to use PC2-6400 RAM in a laptop (do SODIMMs even exist with this speed?) is if you have a X9000 processor which is overclockable and you want to run higher rated RAM for overclocking headroom.