(5/10) Overpair against aggressive opponent

[Paul B.]

Full Tilt Poker, $5/$10 NL Hold'em Cash Game, 6 Players
LeggoPoker Hand History Converter

UTG: $1,164
MP: $988
CO: $2,030
BTN: $1,040
SB: $1,691.95
Hero (BB): $1,179

Pre-Flop: 9[img]/images/graemlins/heart.gif[/img] 9[img]/images/graemlins/diamond.gif[/img] dealt to Hero (BB)
<font color="red">UTG raises to $35</font>, 4 folds, Hero calls $25

Flop: ($75) 4[img]/images/graemlins/spade.gif[/img] 7[img]/images/graemlins/spade.gif[/img] 2[img]/images/graemlins/heart.gif[/img] (2 Players)
Hero checks, <font color="red">UTG bets $55</font>, Hero calls $55

Turn: ($185) 4[img]/images/graemlins/club.gif[/img] (2 Players)
Hero checks, <font color="red">UTG bets $180</font>, Hero ???

Villain is gamble4you, an aggressive winning regular. He's running at 25/18/4.4, turn AF of 3.5, river AF of 3.9. Other than that I don't know much about him. Likewise, he probably doesn't know much about me (I'm new to 5/10).

From his perspective, it's unlikely that I have a set (or a big hand in general). I think he either thinks that I could have a 7 or a midpair and he is putting pressure on me, or he thinks that I could have a flush draw and he is bluffing. What do I do and what's my plan for the river?