Step 2: Dragonology 101
Here's some links to several sources of speculative dragon biology and a bit 'o dragon lore
Here be Dragons! <--Rather ok, lots of lore
D.R.A.G.O.N.S. <-- A wee far fetched, but very detailed and thought provoking
~~~DRAGONS~~~ <--Hasn't been updated in a decade, some good art though
Animal Planet: Dragons <--Oh My Spaghedeity! Awesome! Mostly video however, but gives you an idea how they move
Wikipedia, Dragons <--Good ol' Wikipedia
My Thoughts on Dragonolgy:
Origins? For starters, I feel that dragons, if the did/do exist, would be a highly evolved reptilian animal that either branched off from early theropod dinosaurs, or from early Monitors, or Iguanidae like-reptiles, for these species exhibit many dragon like traits (like, OMS those ickle buggers bite hard!).
2 legs or 4? My guess is 2 legged, 2 winged. However all gliding reptiles use flaps of skin, not wings like a bat. This suggests that it is possible for a dragon to have 4 legs and a set of glider "wings" but I doubt it could get bigger than 1 meter in length. This said, I go for the 2 legged bat winged velociraptor (but bigger).
Fire breathing Easy, flint stones are picked up and carried in mouth, and when fire breathing is desired, they are simply clacked about with the tounge whilst the beast reverse farts (through the mouth somehow, more of a burp really). Alternatively, flammable poison may be spat and lit in the same manner. Poison spitting + fire breath would be pretty awesome...
Coloration As you may know, some reptiles can change color. Much of my life I have watched with fascination as the lizards around my house battled for dominance, utilizing there ability to change color to intimidate there opponents. Color changing is not limited to anoles and chameleons, although I can't find any "official" documention, a wide variety of reptiles and amphibians can change with mood and body temperature. One fairly dull looking lizard, the Texas Spiny (<--fearsome up close), can turn it's underbelly a brilliant blue when excited. So in short, dragons can theoretically be any color.
Food Well, probably not human virgins, LOL i think that was pervy minstrels day dreaming. My guess would be deer, pigs, fish, etc... Feeding habits would probably bee akin to eagles, just larger.
Behavior In my opinion, like a great big bird of prey.
Intelligence High, able to understand most emotions, but probably not capable of speech. May be domesticated and capable of learning complex commands.