If you can sell your iMac on Craigslist for about 80-90% of retail for a new one. Then upgrade whenever you feel like it and can afford it.
There are several in my area being sold for $1400-1800 depending on 2009 i5 or i7.
You'd be paying approx. $300-$400 for an '09 i5 27" -> '11 i5 27" upgrade.
Looking at the price of current year refurbs, I'd say your math leaves much to be desired. Also I've stated this before, but imac used value hinges on the condition of the display. If it looks old and ugly, that kills the resale value.
For the OP, it's time to update when the computer ceases to do what you need in a timely manner. 2013 should be the next really major upgrade from intel. I can't personally wait that long, but my requirements jumped significantly in the past year (or I probably would). We're really past the days where you need to update frequently for most things.
I have a iMac from late 2009 and I was just wondering should I update, or is it fine to keep and upgrade, when would this model be considered old?
If I sell an '09 iMac for $1400.00
A brand new one costs $1699.00
Tax here is 8% which on $1699 is $135.92. $1699 + 135.92 = $1834.92
$1834.92 - $1400 =$434.92
I did mention approx. $400. Which is close to $434.92
For me I never look at refurb prices because I don't want to wait for them to ship one to me.
Check Apple online and OTD price is $1,839.17 w/ estimated price.