2-5 NL flopped set, 4 card straight on river

[onedollars]

2-5 Live game. Opponent has 1200 and is an aggressive, winning player, and views me as the same, though I play many more pots than he does. We haven't tangled in a pot yet, as we are both content to cut up the many fish at the table. I cover him to start the hand.

I have 66 and raise to 20 in MP, 3 callers, including villain in BB.

Flop (80) - 3 6 7 rainbow
Villain checks to me, I bet out 50, fish in LP calls, villain calls.

Turn (230) - offsuit 8
Villain checks, I bet 175, fish folds, villain thinks for about a minute, slowly counts out 175 and calls.

River (580) - 5
Board reads 3 6 7 8 5

Villain bets out 100 fairly quickly. I...?

Comments on all streets appreciated