PowerPC :: IMac G5 - Non-Responsiveness, CPU Usage Maxed Out

I have an iMac G5 (Ambient Light Sensor) that runs Leopard pretty slowly, even with 2GB of RAM. Sometimes, it's a pretty severe lag: Type a sentence, twiddle your thumbs, bam, it's all inputted at once. Sometimes, the whole system is non-responsive for 60-120 seconds, before, bam, accepting all your key- and mouse-strokes at once.

Activity Monitor also reports extremely high CPU-usage. I've been using it for a couple of hours this morning and it's been maxed-out or nearly maxed-out basically the whole time.

There aren't any processes that appear to be hogging resources. Sometimes, it's finder / time machine that's using the most. Sometimes it's Safari or Mail.

I'm open to the possibility that it's just time to upgrade, but I was wondering if the forums had any suggestions on how to troubleshoot or reclaim some CPU cycles.