i think the standard answer for "is the 5400 worth the money" has to do with what you are going to use your PB for.
if you are going to do digital video editing, etc. that requires a LOT of read/write access ... it might be.
if not, then probably not!
if they offered a 7200rpm drive, that would more than likely be worth an upgrade vs. the 5400 ... i haven't seen $175 (or however much it costs) of an increase in performance ...
but, you could have $$ to blow!
why not just walk into the store and buy one! i did tonight!

only thing, they didn't give me the edu discount on the APP ... need to go back tomorrow!
[edit: i heard by replacing a hard drive w/ your own voids some sort of warranty .. not sure what exactly]