As Roli said...you will have more options of keeping everything inside with a bigger case if you want a triple rad. Depending on what you are cooling you may be able to get away with a smaller rad.
Here is my failed attempt at going with an external rad enclosure. I was using a Watercool HTF3 at the time.
http://img.photobucket.com/albums/v3...icparts023.jpg
http://img.photobucket.com/albums/v3...icparts022.jpg
http://img.photobucket.com/albums/v3...icparts041.jpg
http://img.photobucket.com/albums/v3...icparts044.jpg
