You can add a class to each of your .row divs to add some space in between them like so:
.spacer {
margin-top: 40px; /* define margin as you see fit */
}
You can then use it like so:
<div class="row spacer">
<div class="span4">...</div>
<div class="span4">...</div>
<div class="span4">...</div>
</div>
<div class="row spacer">
<div class="span4">...</div>
<div class="span4">...</div>
<div class="span4">...</div>
</div>