Cosi?

codice:
	body{ 
		background-color: gray;
		color: #000;
		font-family: Verdana, Tahoma, Arial, Trebuchet MS, Sans-Serif, Georgia, Courier, Times New Roman, Serif;
		font-size: 11px;
		margin: 0px;
		padding: 0px;
		text-align:center;
	}
	
	#wrap{ 
		background-color: #CCC;
		width: 90%; 
		margin: 10px auto 10px auto;
		text-align:left;
	}
	
	.box-container,.box-container-space{
		border: 1px solid #000;
		background-color: #FFF;
		height: 300px;
	}	
	.box-container{
		padding: 5px;
	}	

	.box-yellow{
		width: 49%;
		background-color: yellow;
		float: left;
	}
	
	.box-green{
		background-color: green;
		margin-left:51%;
	}
	
	.box-blue{
		background-color: blue;
		width: 49%;
		float: left;
	}
	
	.box-red{
		background-color: red;
		margin-left:51%;
	}
br{
	clear:both;
}