Welcome to the forum.
First off do you have a good reason for mounting the radiator to the top of the case? I would advise against it if you have any other options.
It can be very difficult to properly bleed a system if you have the radiator mounted at the top of the case. Also it makes the use of an in-case air trap almost impossible as there will be a higher point in the loop.
If you must mount it at the top of the case you really have two options. If you want the coolant temp to be lower have the radiator sucking in from the top. If you want the system temp to be lower have it blowing out the top.
The convection properties (heat rising) will make the rad hotter than if you had it mounted else where. However because of the convection if you have the fan blowing air out it will increase the over all airflow through your case.
If you want alternate suggestions for mounting places let us know what kind of case you have and post pics if you can, and we can give you some more efficient placements
