Just stumbled on this, so I'm late to the party. I have an iBook 14" G4 1GHz bought at the start of 2004 that's still working wonderfully. I've replaced three batteries (the weakest part of the computer IMO), replaced a dead hard drive a couple of years ago (to be expected, in a laptop especially), and the E key label is worn off. But it just keeps ticking away, has hardly been off in all these years.
But I'm not sure I would make any more major investments in it only because the PPC processor is now an orphan. The OS can't be upgraded past Leopard, other Apple software will no longer be available (of course they have a vested interest in "encouraging" you to buy a new Mac!), and many software publishers are starting to create Intel-only upgrades, which might leave you with unpatched vulnerabilities. If your iBook is just a secondary computer, then you might want to make small repairs to keep it running. But if this is your primary box, then you are going to fall farther and farther behind the rest of the Mac ecosystem; and, as another poster suggested, you might consider getting a new MacBook (or MBP if you can afford it). Good luck.