What's the right time to upgrade Mac?

I has been saying many times that I plans to replace my 3-years old iBook 500 with Powerbook. It is just that I am finding a rigth time to do so. When is the right time? Now? Not really. The buyer's guide from macrumor.com indicates that Powerbook is approaching the end of life cycle. "But when should I upgrade?", I periodically ask myself. These days, I read the most recent rumors and conclude the best time is:

• New series of Powerbook (probably G4 Dual or G5) rolls out
• Mac OS X 10.4 is shipped

At the moment I am using Panther. So having the new Powerbook with new OS is the best upgrade plan. But I think the time should be around or after X'mas, meaning that I will need to wait 9 more months...