As was previously stated, the HD rule is 3.0. It was erratta'd in
Tome and Blood.
As a fighter/wizard who had access to
Polymorph Self in a previous campaign, and examined it from every possible angle, let me say that it's strong, but not as overpowered as some people seem to think.
First of all, a dragon doesn't wear a lot of magic items. Cheesy dragon splatbooks aside, you'll probably get a couple rings, a headband, maybe bracers. Hardly the plethora of magic item slots a standard PC has. And any items that cannot be worn by the new form are absorbed and cease to function. That's hit number one.
Dragons don't use weapons. So a fighter-type who's invested in Weapon Focus, Weapon Specialization, etc. is going to lose the use of some of their feats. Hit number two.
You don't get any extra hit points (though you do heal up a bit). So you are a load weaker than an actual dragon. Even worse, depending on whether or not your DM allows dragons to wear amulets. (And even if your DM
does allow it, the red dragon is going to figure something's up when he sees a gold dragon decked out in biped trinkets!)
As a powergamer, my character hardly ever changed into a dragon. You simply can't get a strong enough dragon that's at your wizard's caster level. (And my fighter/wizard was a homebrew class that had a full wizzie's caster level, so I was playing with the full range.) He only did it when he needed to
fly for long periods, and didn't have
Fly handy. Or to show off.
The most powerful (possibly abusive) way to use
Polymorph is to change your party fighter into a troll or stone giant or annis hag. Humanoid forms let your fighter use all his equipment, plus gain serious strength and natural armor. But dragons? Don't sweat dragons at all.
And the instant any intelligent foe sees a group of adventurers, one of which is a troll/giant/annis hag, guess which spell will be first on it's casting list?