I have three identical Intel Core Duo 2 GHz 20" iMacs each with 2 GB of RAM which I bought at the same time about 1.5 years ago... I plan to sell one, but I don't know which of the three I should sell... they all look and run just great... so here's the probable dumb question.... is there any way of determining which one has the most wear on it? What would be a reasonable selling price today?