[Radarbob]

Put it off till it hurts. You'll get more bang for your buck when you do.

For example, just this week I replaced my original iBook after 6 years. I bought a used 900MHz iBook. Despite being "obsolete" look what it has that my Precious does not:
*CD burning
*DVD reading
*higher screen resolution (800x600 vs 1024x768)
*significantly better video memory (4MB vs 32MB)
*faster buss and memory
*USB 2
*firewire

Now, think about the difference between the my "obsolete" 900MHz iBook and the current model. The current model is faster. But that's about it. A little bit extra speed just is not worth a thousand dollars, IMHO.