What are my options for a 7200 RPM HD?

Hello all. I have a powerbook with the factory 100gb 5400rpm drive installed. Although i have recently did a fresh install of 10.5.6 and updated.

I am getting a little worried about space and speed issues as i am using this machine to trigger samples live and other smaller audio projects. I would like to either install a larger faster drive or a larger drive just to try and keep the amount of variables of something going wrong down at least by one.

I have heard that a drive starts to 'degrade in performance' sometimes when as little as 50% of the drive has been written to and although i am not actually writing to the disk in a live situation the disk is still spinning...

I have written the longest post known to man :. Anyway, thoughts?

Thanks in advance!