What line with middle pair here?

[Unarmed]

$10 buy-in, table is insanely tight

Party Poker No-Limit Hold'em Tourney, Big Blind is t300 (6 handed) converter (http://www.selachian.com/tools/bisonconverter/hhconverter.cgi)

CO (t1355)
Button (t2170)
SB (t435)
Hero (t1350)
UTG (t1390)
MP (t1300)

Preflop: Hero is BB with J/images/graemlins/club.gif, 4/images/graemlins/heart.gif.
<font color="#666666">1 fold</font>, MP calls t300, <font color="#666666">3 folds</font>, Hero checks.

Flop: (t750) 9/images/graemlins/club.gif, 4/images/graemlins/club.gif, 2/images/graemlins/heart.gif <font color="#0000FF">(2 players)</font>

[swarm]

push or fold are your only options, any kind of savvy player will push all in on any min bet. If you bet 600 and he has hit your committed anyway, so you push or check depeding on your read.

The fact that he just called from middle position is odd to me. Would he limp an overpair?

[Insty]

Unless this guy has been limping every hand.
Keep check/folding until you can beat Aces, then push.

Don't lose your stack on a pot you never wanted to be in in the first place.