In my opinion, hold off. Do them in the order they where released (Dead Money, Honest Hearts, Old World Blues and then Lonesome Road). The DLC in this game have an overarching plot which you'll enjoy ten times more if you do it in order, become invested in the situation and learn about the characters, themes and ideas. I know you'll have to wait a while but New Vegas is a big game, plenty to do! If you're looking for what to do in the main game, my favourite part is the companion quests, go find Veronica, Boone and the others and learn about them and their quests. Also, if you know more about the world and characters you'll notice little nods to them in the DLCs.