Question about the oldest redeemable packs.

I played a couple years ago, I think, but it was a while ago. Relatively recently started playing again, and I noticed I have a few HeartGold SoulSilver cards, but I don't see any way to get more, as I don't think they have a PTCGO code in the booster packs, and they're too expensive to buy and check..

Also, I have a Holo Double Colorless Energy from that set that seems to be worth a ton, as the public trades for it are insane, being dozens of packs and EX's and such. Why is it so sought after?

EDIT: Thanks for the answers, people! Now I just have to find someone who wants to buy it, as I don't play the game that often... :)