I perfer keeping everything inside the case just for the sake of neatness. I would get a Hayden Radiator if I were you, they are by far the best. People rarely call them by their names they just seem to denote a size a be done with it, most common size is the 4x4" verison. I use one of those myself and it works let me tell you!. But they make other sizes as well. If you want to external mount it, check out how the people at SwiftTech do theirs,
http://www.swiftnets.com/barebone.htm . Another fine radiator is the DangerDen cooling cube,
http://www.dangerden.com/product_pages/radiators.htm . They also sell larger models of the Hayden 4x4 Radiator. The only problem with the small 4x4 Radiator is that they are hard to find sometimes. I got mine at
http://www.millisec.com . When you get a 4x4 radiator from them @ $45 bucks you also get a nice Sanyo Denki 92mm fan with chrome guard. Sanyo Denki's are only second to Panaflo (my opinion) because they are super queit and move a considerable amount of air. Also just for reference if you do get the 4x4 radiator, those are the same used in just about every watercooling project on procooling dot com, so that has to tell you something