The 50cfm fan would move the most air, but be the noisiest. The 26.2, 4 pin would be a little quieter and cost a little less. The 3 pin version most likely will be able to have it's rpm monitored and plug into one of the mb connectors. For case airflow, the 50cfm is the way to go.
As for the weight issue, I've heard that when you have a very heavy fan mounted and the mb vertical, it can cause the hsf to sag and not make good contact with the die. While I seriously doubt this would be an issue it's something to consider. And yes, if you move the computer around a lot it can knock the hsf loose and go crashing through the computer more easily. But once again, I doubt this would be an issue, but that's why AMD has the weight limitation. I have an mc462 which weighs twice what that one does, but it mounts with bolts through the mb, weight is not an issue at all with it. However, free space around the cpu is and I had to modify it to make it fit my Asus a7m266. Make sure you do a little research before you buy, but Tom's rated it to be the best cooler on the market right now...
http://www.swiftnets.com/MC462.htm
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