URGENT: Sending divs behind each other

[AngryBirds]

#header{
	background:url(images/header-background.png) no-repeat;
	width:100%;
	height:100%;
}

#curve{
	background:url(images/curve.png);
	width:906px;
	height:358px;
	position:relative;
	top:30px;
}

#curve-down{
	background:url(images/curvedown.png);
	width:906px;
	height:427px;
	position:relative;
	top:85px;
}

And

	<div id="header">
		
		<div id="curve">		
			<div id="curve-down">
			
			</div>
		</div>
	
	</div>

I want to send #curve on top of #curve-down. i tried z-index but it didn't work eighter. somebody help me please. i need an urgent help please.