It's been three and a half years since I bought my last desktop computer, a 24 inch aluminum iMac. At the time of purchase it had decent specs, and it's held up tolerably in terms of performance and what I generally use it for. It was a good value buy.
However, I was reading some rumours that the next refresh of this series shall be announced soon. [Update: The refresh happened. Quad-cores!] Looking back this would make it five iMac revisions (including this prospective one) that I've passed through. None of the subsequent upgrades have been revolutionary in terms of design, but cumulatively they make my current machine look rather plodding. Moore's law continues apace, even for pricy Apple products.
Upgrading would allow me to roll my machine down to my parents, who are still underutilizing an even older white iMac from the year prior...
And if I just wait till later this year, the new iMacs will ship with OSX Lion. Four years is a pretty good stint for a computer, don't you think? The alternative would be to wait another year or so, and stick it out until the five year anniversary. I don't need a new machine, it would just be exceedingly nice. I'm not convinced that's a compelling enough reason to upgrade.
More speed! More cores! More RAM! More hard drive space! Shiny! I can feel the itch already. I suppose the operative question should be, "Is this machine going to last me four years? Am I going to be happy on this thing four years from now?"
There's also the opportunity cost of the roughly three or four grand I'd probably budget for this. Hmm... I'll have to think about this some more.