My wife is still using her 2004 20" G5 iMac. It's developed a number of problems... It's definitely time to retire it. I just pre-ordered her a 21.5" 2.9GHz iMac. Will be upgrading the RAM from 8GB to 16, as soon as they can tell me how much that will cost.
In fact, our newest Mac is a 2006 MacBook Pro, which I'm still using, Other than some worn off letters on some key caps, and a swollen battery (which resulted in a mouse button that stayed jammed "on" -- took me quite a while to figure that one out), it's still going strong.