If your dead set on upgrading to a new bike I would wait to to do anything with your old one until you have found it's replacement. On many bikes the stock wheels are one of the weaker points in terms of spec and one of the better upgrades you can make is a new set. In this case what I would do is replace the new bikes wheels with a nice upgraded set and move the stock wheels from the new bike over to the 1500 to sell. That way you get a nice upgraded wheelset on your new ride and the 1500 has a decent usable set on when you sell.
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Road: Quattro Assi Scandium w/ SRAM Rival and Rolf Echelon's
Mountain: 2005 Specialized Enduro Comp
2003 Gary Fisher Tassajara