To put it gently: Relatively few PowerPC computers have enough horsepower to do this well, especially if you're planning on recording anything else that might be "heavy" or might incur penalties on subsystems that screen recording will rely on, like, well, all of them.
VNC to a more modern machine who will only be doing the recording is one option. Another, probably better, option would be to get a really new computer (Sandy or Ivy Bridge or better) and use one of those USB video or game capture devices that accepts HDMI/VGA/DVI and run y our Mac through that for capturing.
If extremely low resolution (literally 640x480) is fine for your needs and your Mac has an analog output option, you may be able to consider recording to a DV/miniDV/digi8 camera or to an outboard settop DVD recorder and then imovie or handbrake the footage back in.
Here's what it looked like when I did it with my Pismo and a Canon GL2. I'm mostly sure this was done directly with a single S-Video cable.
https://www.youtube.com/watch?v=Z-UslsOQzdg&t=5s
Unfortunately, I don't know of any stand-alone HDMI/DVI/VGA recorders. Most work these days involving recording a computer screen involve a modern PC/Mac running a capture card, or a computer recording its own screen, which the modern CPUs and GPUs are basically good enough at doing on their own. (I have a ho-hum business desktop from 2011 with a modest GPU added in from four years ago and it's able to record and stream out its own display with a game running, with minimal trouble.)