So my Averatec (Which I hate so much because they refurse to support it) had a 60GB 4200RPM harddrive. I recently ordered and installed a 100GB 5400RPM harddrive w/ an 8MB cache. This made a HUGE jump in performance for my machine (also added some ram which helps). HDTach reported something akin to a 4x jump in burst speed (from 25mbs to 93mbs). I also ripped 30 minutes of DVD which previously took 10 minutes, in just 2 minutes.
Only problem is that after re-installing to the new drive — I needed to move everything older over (which wasnt a problem with and IDE->USB adapter). My pictures was easy (picasa doesnt really care where the pictures are). Itunes is a pain because you have to copy over the music, add it to your library and then consolodate it. You loose your play count and play lists — sigh. Firefox is cake. Thunderbird is easy, just copy over your profile from the old drive to the new and install thunderbird. Slype keeps the buddy lists online since version 1.1 (I belive) so thats set also.
A note with Itunes, make sure you de-authorize your old machines, but if you dont its not the end of the world. You can only have 5 machines authorized at a time (as I did), but you can fix that. Login to the music store on your new machine, goto your accounts page and you will see a button to “”Deauthorize all computers” — hit that and you are reset. This only works once every 12 months (one year) so watch out.