I'm using a Palo Alto PA-450 on a couple of systems in my house, and it has ~60mm clearance for a HS/F total. I am using the stock HS/F from an Athlon XP 1600+ in one system (Athlon 850 CPU) and it works quite well. I am using an Alpha PAL6035 with a 15mm Adda fan in the other, and it works pretty well (Athlon 1200/266 CPU, which runs pretty hot). Now, the one thing that has to be mentioned is that I am using the stock AGI 250w power supply that comes with this case, and that PS is ventilated on both the back and the bottom, so the PS fan (replaced with a Panaflo L1A) helps to exhaust excess heat from the CPU, something that wouldn't happen with a PS with a non-ventilated bottom. You could also use an Enermax dual fan PS to do the same thing, though it would be louder.
Pete