(NL600) Passively played AA faces river overbet

[LouisCyphre]

SB is 51/26/3, obviously fishy and capable of bluffing
MP is 24/18/4 and probably decent

Party Poker - No Limit Hold'em Cash Game - $3/$6 Blinds - 6 Players - (LegoPoker HH Converter)

SB: $974.45
BB: $398.30
<font color="black">Hero (UTG): $679.00</font>
MP: $637.05
CO: $717.25
BTN: $935.60

<font color="black">Preflop:</font> Hero is dealt A[img]/images/graemlins/diamond.gif[/img] A[img]/images/graemlins/spade.gif[/img] (6 Players)
<font color="red">Hero raises to $21.00</font>, MP calls $21.00, 2 folds, SB calls $18.00, BB folds

<font color="black">Flop:</font> ($69) 4[img]/images/graemlins/diamond.gif[/img] 3[img]/images/graemlins/heart.gif[/img] 2[img]/images/graemlins/diamond.gif[/img] (3 Players)
SB checks, <font color="red">Hero bets $50.00</font>, MP calls $50.00, SB calls $50.00

<font color="black">Turn:</font> ($219) 5[img]/images/graemlins/diamond.gif[/img] (3 Players)
SB checks, Hero checks, MP checks

<font color="black">River:</font> ($219) 4[img]/images/graemlins/club.gif[/img] (3 Players)
<font color="red">SB bets $334.00</font>, Hero ???

I checked the turn for pot control reasons as there could easily be 22-66 or a flush out there.