Well Congrats on choosing a VERY good heatsink, the Artic Freezer 64 Range are fantastic - as long as you have an ambient case temperature between 20-30Celcius you should have no problem running your fan @ 1000RPM and keeping max temperatures below 50C. EASILY!

I have the Artic Freezer 64 (Non Ultra) - no fan adjustment and mine runs at a constant 1860RPM (Max RPM i've seen it go) and thats with a 600Mhz overclock (2.0Ghz -> 2.6Ghz) and my temps don't go above 54C - ever (max ambient case temperature of 30C depending on weather).

As for your CFM query 18CFM is NOT a problem as long as you have a set of good case fans (that have equal CFM settings)

- Intake at front
- Out take at back

So feel free to go ahead and shut it down to 1000RPM, however.. there really isn't any need the Artic Freezer 64 is INCREDIBLY silent as a cooler, if you have any noise coming from your case it will be from your graphics cards heatsink/case fans/psu.

I recommend: Vantec Silent Stealth Fans (60mm/80mm/92mm/120mm) ( no case really needs more than 2 fans as long as they are reasonably powerful - even in large cases.

If your looking for silence, i suggest Acoustic matting for example: Akasa Acoustic Matting deadens vibration of fans / hard drives / case in general.

P.S if you find the 2500RPM setting loud, it's not the Heatsink&Fan it's your case vibrating or something else such as graphics card..

I have the Artic Cooler Freezer 64 which is the same as the Ultra except you have a fan controller, and the Freezer 64 @ 2500RPM makes a total max noise of 1 Sone (nothing at all..)