Hi chinkee,
I am using a SLK-900A in "TheMoneyPit" rig in my sig. I have a pretty good overclock going on as well.
I am using a 92mm Panaflo medium speed fan. 48cfm @ 32db, 2400rpm I believe is the rating. It is not loud at all. This fan keeps my CPU within 2 degrees of the case temp at idle and within 6-7 degrees of case temp at full load. Can't ask for much more than that! Make sure you use a good TIM like AS5. (or any of the AS family...much better than white paste from Radio Shack)
The Tornado can't be beat if you absolutely could care less that your case sounds like it's about to levitate off the desk and you're massively overclocking to the point where it's not really stable. The Tornado puts out enough air to fill a hot air balloon in about an hour.
I think the Tornado comes with a speed control, IIRC...don't quote me on that though. Pair a Tornado with a speed controller and you've got the best of both worlds.
I am happy with my setup, FWIW.