I'm gonna be the black sheep here and say DO IT!
My experience is twofold. My first Mac laptop (which I owned for only a few weeks) was the G4 1.42 / 14" iBook. While usually pretty snappy, it was a dog whenever you let the HD fall asleep. It felt sluggish and unresponsive when multitasking (though I only had 512Mb of memory in it). I only realized long after I got rid of it that it could be sped up.
The second case experience is my Clamshell, which I'm on right now. Replacing the original 4200rpm 10Gb drive has given this machine impressive chops for a 10 year old laptop. I'm streaming music over the AirPort, surfing in Safari with three tabs open, and running Twitterific, and everything is running great. This wasn't possible with the old hard drive (the memory had been maxed out long before this).
I think a bigger hard drive would be better if you can get one. I got a 160 for my Clamshell, even though I can only use 128 of that. But if you replace the drive and disable the "Put hard disks to sleep when possible" option under Power Management, I think you'll see a big improvement.
And the job to pull the drive isn't
that bad. I did the Clamshell in just over three hours, and it's supposed to be the worst laptop ever designed when it comes to upgrades.
http://www.ifixit.com/Guide/Repair/Installing-iBook-G4-14-Inch-933-MHz-1-33-GHz-Hard-Drive-Replacement/189/1 - You'll be needing this.
