You can also get 7200 RPM drives which are significantly faster again. I'm not sure of the warranty issues, but doing the operation yourself is fairly straightforward.
Apple authorized repair shop must install new drive or Warranty Voided.
Do not waste time or money on 5400 drive.
60GB 7200rpm IBM/Hitachi drives $269 online; solid results, excellent idea.
Install old drive in external case USB 2.0 / Firewire for added external memory.