That really depends on what you want to do, computing-wise.
I think if you just do email, word processing and light web surfing, that computer would be fine. You might consider upgrading it to run OS X Tiger 10.4 for a speed boost, assuming you have enough RAM.
If you need your laptop for applications that use more resources, then I'd recommend you get a new one so you can run OS X Leopard (10.5) or the upcoming Snow Leopard to get the advantages of the new operating systems. You can always keep the TiBook around for something nifty too, like running home automation software.
If your main concern is being able to get help with a computer that old, I think you'll find people can still assist with it. Upgrading to OS X Tiger will help that situation.
