It'd be a decent performer under OS X. The main limiting factor would be the low amount of VRAM. Also, unless this machine has already been upgraded to a G4, the upgrade will push up the price a bit. Depending on how much you might be spending, it might be cheaper overall to get an older G4 tower.
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PPC Macs:

• Power Macintosh G4
  
• B&W Power Macintosh G4
  
• Power Macintosh 6500/225