Bottom Two Facing Heavy Flop Action

[Double Eagle]

This hand is from my local card club's weekly three table $100 tournament. I am a regular in this game and am probably one of the top two or three players in the field. We are still in the first orbit and everyone involved still has right around their starting stack of T4000 with blinds at 50/100. The structure of the tourney is very much a crapshoot, which is fine with me as most of the players are loose early and weak tight late.

I am in the SB as 3 players limp to me and I complete with 5c6c. The BB, who is not very good, makes it 300. Everyone calls as do I, so there is 1500 in the pot as the flop comes 5s6d8d. I check, expecting to be able to check raise what I hope is the BB's big pair. He bets 500, one player folds and then a player that I don't know (but who seems to know what he is doing) raises to 1500. The button folds so it is back to me. What's my action?