If I had Jaguar and my Cube with me I would do a fresh install and erase the Hard Drive.
If you have done any moderate/heavy work with your computer (lots of photo/video editing, using/installing a lot of applications) since you last erased the Hard Drive, save everything on CDs, on another Hard Drive, or another computer, and reformat. Most likely you will end up with more speed than you would if you just upgraded. Plus, you can get rid of all the clutter that would slow the computer anyways. Just remember to back up EVERYTHING.