G5 now running S L O W E R

Question

Over the past 4 months (and after upgrading to 10.5.8), my overall G5 performance speed has slowed by over 50%. Everything from finding and opening files, to Starting Up (power off to Desktop can take 10 mins!) or loading new pages in Safari, is running MUCH slower.
Granted the hard drive is full of data (6GB out of 372GB is free space), and that may be the problem, but I wanted to see there are other potential problems before I start purging data. (Yes, it is all backed up through Time Machine.)
If the full drive is the problem, is it possible to replace the original drive with a larger internal one, and transfer the data intact? If so, is this procedure recommended for endusers to do, and is it documented somewhere?
Thanks.

Answer 1

I have not been able to complete my Super Duper restore.
My steps so far:
Created my Super Duper Backup All Files clone from my current internal drive to an external drive.
Replaced my internal drive with a new drive and added the second drive
Restarted my machine using the install disc
Formatted the new drives (Mac OS Extended (Case-sensitive, Journaled) using Apple Disc Utility
Ran Verify Disc and Repair Disc on the new internal drives
Ran Verify Disc and Repair Disc on the 0external drive containing Super Duper clone
Successfully restarted my machine from external drive containing Super Duper clone (to verify that it is complete)
Began restore from the external drive containing Super Duper clone to the internal hard drive
This restore progressed for 1-2 hours, then stopped with the message:
Restore Failure
Could not restore - input/output error
I restarted the machine and repeated the restore process, but got the same error message.
Any suggestions what I can do to complete this restore?