$collection = Mage::getModel('catalog/product')->getCollection();

note this is the long way of doing this:

$collection = Mage::getResourceModel('catalog/product_collection');

      

  get the attribute set that you're after

//load By Name
$attributeSetName = 'Default';
$store = Mage::app()->getStore();
$attributeSet = Mage::getModel('eav/entity_attribute_set')
->setStore($store)
->load($attributeSetName, 'attribute_set_name');
$attributeSetId = $attributeSet->getId();

      

  of course you could do this in one line
  but remember that you're loading a whole entity here
  so if you need to use any other data from the
  attribute set, it's probably not optimal to run this every time

$attributeSetId =  Mage::getModel('eav/entity_attribute_set')->setStore($store)->load($attributeSetName, 'attribute_set_name')->getId();

   

addFieldToFilter only works before load.
if you need this to occur after you've loaded it
you can call     

//$collection->resetData();
// this should only be used if you've already loaded the collection
$collection->addFieldToFilter('attribute_set_id', $attributeSetId);