Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
I generally replace every two years ( I guess it depends on the changes, 09 to 10 was not big but the 2001 is significant). Just sold my 2009 Imac 27 for close to 1900 and bought a new 27 3.4
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.