Didn't see an answer to your question and I don't want to discourage new users.
If you put your PC into a closed desk, the heat in the desk may be 100F or more. This means that a really good heat sink has to make up over 30F over ambient room air. Well, there is NO possible way an air type heat sink/fan will manage this differential at any price. Water CPU cooling may do better, but it still is limited to air that is 100F. Water-cooling can only cool to the air temp in a 100% efficient system, and they aren't.
So, my answer is to put a vent and fan on the backside of the enclosure so you have to PC in close to ambient air temps and you're all set. A stock Dell is a very well engineered system for PC cooling. Just give what you have a chance to work, and it will.