Which Thermalright heatsink model are you thinking about? (oops, sorry for the mysterious bolded URL, I pasted the wrong thing

). The only one I've heard had a fit issue was the SLK-900, the biggie... and even then, if you want to buzz a little off of some fins to provide capacitor clearance, you're ok.
Smart Fan II draws an unreal amount of amperage, so make sure not to plug one of those into your motherboard. Power it from your power supply. A YS Tech adjustable-RPM 80mm might be a better pick unless ~50cfm isn't enough for you.