$5 SNG - tough spot w/ top pair

[The B]

Full Tilt Poker, $5 + $0.50 NL Hold'em Sit n' Go, 80/160 Blinds, 3 Players
LeggoPoker.com - Hand History Converter

BTN: 2,937
SB: 2,550
Hero (BB): 3,513

Pre-Flop: (240) 7[img]/images/graemlins/club.gif[/img] K[img]/images/graemlins/club.gif[/img] dealt to Hero (BB)
BTN folds, <font color="red">SB raises to 320</font>, Hero calls 160

Flop: (640) 4[img]/images/graemlins/diamond.gif[/img] 7[img]/images/graemlins/diamond.gif[/img] 6[img]/images/graemlins/club.gif[/img] (2 Players)
<font color="red">SB bets 640</font>, Hero ???