Villain's pretty bad. Bet turn harder? How much on river?

Full Tilt No-Limit Hold'em, $0.50 BB (6 handed) Hand History Converter Tool from FlopTurnRiver.com (Format: FlopTurnRiver)

Button ($50)
SB ($20.40)
BB ($75.60)
Hero ($75.15)
MP ($49.75)
CO ($55.25)

Preflop: Hero is UTG with K, K.
Hero raises to $1.75, 1 fold, CO calls $1.75, 3 folds.

Flop: ($4.25) Q, 3, 7 (2 players)
Hero bets $4, CO calls $4.

Turn: ($12.25) 2 (2 players)
Hero bets $8.5, CO calls $8.50.

River: ($29.25) 5 (2 players)
Hero ???