the larger the screen, the more value they hold.
a 24" is going to hold value a lot better than a 17"
because of the novelty of a huge screen.
Still, they don't hold value as well as laptops, that's
for sure. It kinda depends how LONG you need it to
hold value for. In the first year, it will significantlly
decrease in value, but in 3 or 4 years it will still have
a fairly high value for that old of a computer.
I just unloaded my 4 year old G4 iMac for a pretty
high-spec'd newer PC desktop.