I consider 3 years to be the maximum for running current applications on a PowerMac. 1.5 - 2.0 years on a laptop computer.
I have 3 machines which I consider still useful:
PowerComputing PowerCenter/225 with Mac OS 9.x
Apple PowerMac G3/800 with Mac OS X v10.3.5
Apple PowerMac dual G4/800 with Mac OS X v10.3.5
The PowerMac G3/800 was a G3/400 and was on the verge of being eliminated from software updates. (If you doubt that, look at the last QuickTime update.) I wouldn't consider such a machine to be useful since it doesn't fill a purpose that can't be done by something else I have. The PowerCenter is still around because I have a lot of SCSI-only equipment that works just fine.
An original iMac with 233 MHz G3 is pretty much done for anything but running old stuff. Even for web browsing, my dad's G3/350 is done because plug-ins and browsers haven't been updated for a while but content has been updated past that point.
The only thing you can do is to buy a processor upgrade, which probably would have been good two years ago, but isn't going to help much now.