Here is a fiddle of what you described: http://jsfiddle.net/fvGMb/
You need to have rows with a set width and then you can have columns that go in those rows with percentages that describe the width you'd like them to be. The columns need to be floated.
CSS
.col {
float: left;
}
.row {
width: 100%;
}
.half {
width: 50%;
}
.third {
width: 33.33%;
}
HTML
<div class="row">
<div class="col half">
</div>
<div class="col half">
</div>
</div>
<div class="row">
<div class="col third">
</div>
<div class="col third">
</div>
<div class="col third">
<div class="col half">
</div>
<div class="col half">
</div>
</div>
</div>