Worthwhile... well, that's a pretty relative term. What do you mean by worthwhile? Obviously, the system won't perform (fully upgraded) anywhere close to a PowerMac G5. The highest I see for a fully upgraded G3 on Xbench's user submitted section is 77.44. The highest score for a PowerMac G4 (Gigabit Ethernet model with dual processors) is 153.51. And, the highest score for a PowerMac G5 is 320.09. (Obviously the higher the score the better) So, as you can see, with each iteration of the PowerMac the score nearly doubles.
Basically, what I guess I'm trying to say is; you can get a G3 B&W and do everything under the sun to it, but it won't perform nearly as well if you would've done the same stuff to a G4, or a G5 for that matter. So, since you're not really going to be doing a lot of gaming, but you are going to be doing word processing, internet, etc. I'd recommend the G4. I haven't been using Macs for very long (Less than a week, as a matter of fact); but I can already tell that the G4 is quite a 'bit faster than the G3, just because of the integrated AltiVec. Everything is just quicker overall. The webpages scroll easier, it keeps up with my typing better, programs work quicker, etc.
So, if you can afford a G4, I say go for it. But, if all you can afford is a G3 B&W then that works, too. Just be prepared to spend more money upgrading the G3 than you would the G4, to make it bearable for everyday use.
Sorry about the ridiculously long response; but, I hope it helped you in some way.
