Just a guess, but since you are getting continuous beeping, it most probably a Video or memory problem. Video has been ruled out with your swap test. And since it only worked after you swap out the memory then most probably during your HSF installation you may have somehow damaged the memory components of your motherboard. I know most boards have the memory and CPU very close to each other and installing HSF require quite a bit of force.
I damaged my own mobo recently after cleaning and reinstalling my HSF and all care have been taken to avoid any damage to anything.