DDR2 speed questions! Insight appreciated!

[MiG29TangentBoy]

Okay, stupid question from someone ignorant about DDR2 (being buried by loasd of DDR here at home, I just haven't fiddled with this kind of memory just yet..)...why are there two different options under DDR2-667 at the Egg...PC-5300 and PC-5400? Is this slight quirk going to make a difference? I'm just slapping a budget AM2 system together for a friend's co-worker and I just want to know that there's no reason to care about such a seemingly oddball detail. Thanks for looking, I appreciate any insight you can give me (and apologize for the inanity of a question my overstressed holiday season-addled brain should be able to figure out on my own).