I just did the same thing, the last set I played was Gym Heroes. I just came back about a month ago when my 8 year old daughter discovered my old binders in the closet and wanted to learn the game.
I can save you some time. Out of your old cards pull all of your Energy and then Pokeball, Plus power, Potion, Energy Search, Switch, Mysterious Fossil, Warp point, and Master Ball. Everything thing else goes back into the binder or box. Now forget about the other cards and sets between the last set you bought and the Diamond and Pearl set, they don't really matter much to a returning player. Everything you will need is in Diamond and Pearl.
Start buying the Diamond and Pearl Preconstructed decks like the Magmorter deck, and you will soon have a solid base of the new trainers and some decent cards to start building new decks.