When do you call down after being raised on the turn?

[wyrd]

I hate giving away money, and this seems to happen a lot. I have top pair, and on the turn someone all of the sudden becomes aggressive and raises me. Do I just call down at that point? Here's are two hands where this happened;

Ultimate Bet 0.25/0.50 Hold'em (9 handed) FTR converter on zerodivide.cx

Preflop: Hero is SB with 6[img]/images/graemlins/club.gif[/img], A[img]/images/graemlins/heart.gif[/img].
<font color="#666666">4 folds</font>, MP3 calls, CO calls, <font color="#666666">1 fold</font>, Hero completes, BB checks.

Flop: (4 SB) 9[img]/images/graemlins/club.gif[/img], 3[img]/images/graemlins/diamond.gif[/img], A[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(4 players)</font>
<font color="#CC3333">Hero bets</font>, BB folds, MP3 calls, CO folds.

Turn: (3 BB) T[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">Hero bets</font>, <font color="#CC3333">MP3 raises</font>, Hero calls.

River: (7 BB) Q[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(2 players)</font>
Hero checks, <font color="#CC3333">MP3 bets</font>, Hero calls.

Final Pot: 9 BB

Ultimate Bet 0.25/0.50 Hold'em (9 handed) FTR converter on zerodivide.cx

Preflop: Hero is CO with Q[img]/images/graemlins/heart.gif[/img], A[img]/images/graemlins/heart.gif[/img].
UTG calls, <font color="#666666">1 fold</font>, MP1 calls, <font color="#666666">2 folds</font>, <font color="#CC3333">Hero raises</font>, <font color="#666666">3 folds</font>, UTG calls, MP1 calls.

Flop: (7.40 SB) 5[img]/images/graemlins/club.gif[/img], Q[img]/images/graemlins/diamond.gif[/img], 8[img]/images/graemlins/heart.gif[/img] <font color="#0000FF">(3 players)</font>
UTG checks, <font color="#CC3333">MP1 bets</font>, <font color="#CC3333">Hero raises</font>, UTG folds, MP1 calls.

Turn: (5.70 BB) 4[img]/images/graemlins/spade.gif[/img] <font color="#0000FF">(2 players)</font>
MP1 checks, <font color="#CC3333">Hero bets</font>, <font color="#CC3333">MP1 raises</font>, Hero calls.

River: (9.70 BB) 7[img]/images/graemlins/diamond.gif[/img] <font color="#0000FF">(2 players)</font>
<font color="#CC3333">MP1 bets</font>, Hero calls.

Final Pot: 11.70 BB