I have an X2, it's cheap, quiet, dropped my temps a good 10*c idle, and about 10-15 load. My VREG temps have gone up, but considering that I'm now able to put more voltage through reliably, and get a higher overclock, and the VREGs never go above about 65-70, then I'm not worried.
The stock cooler was a signle slot design, heavy, really heavy, but with one loud ass fan. To have it overclock to 600/666, I had to have the fan at 100%, which was just too loud for my liking. I now have my fan at 100%, and have reached near enough 700/700, and can use 1.4V reliably.
Case airflow is important, but you've got enough, it's mostly intake that really affects it. Bad intake fan, not enough cool air coming in, can really hinder the X2. It's what happenned with my new case, I used an old 80mm fan, airflow went down dramatically. Once I had my new fans, airflow increased again, and the X2 performed much better.