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Illustrious
You need to read the post, OP is hoping to replace his PC5300 with PC6400 and hoping the new, faster RAM will run at 800mhz. instead of his old 667mhz. RAM when he installs it. OP's question is simple, if it will, upgrading to PC6400 certainly will make his sytem faster. It comes down to what dividers his Dell BIOS is equipped with and will those dividers allow his new PC6400 to run at 800mhz.. Op has no control of his BIOS, it's a Dell. OP knows that. Of course if OP installs PC6400 and the RAM defaults to PC5300 667mhz. there would be absolutely no improvement and it would be a complete waste of money. If you read OP posts, his manual says his system supports PC6400. Hopefully that means supports PC6400 at 800mhz. (with the proper divider provided by BIOS/Chipset and PC6400 DIMMS also cooperating). I have never owned a DELL so in that regard, I know little about the company' systems, BIOS limitations, upgradability path, etc.