How quiet do you want to go?
I have a quietish w. very effective aircooling setup (modded Sonata w. 3rd 120mm fan drilled into the side + acousticpack sound damping material throughout, Zalman 7000Cu running at 12v, P4 3.2, 6800 GT). I get a low whoosh from the rear 120mm fan (only one turned up much, the others use the psu fan controller which never spins them fast), and a whine from the geforce (which I will improve with an NV 5 in a little while). I can't really hear the psu, zalman, hd, or even the cd rom drive (if I keep case door shut).
Looking at your setup I wouldn't worry too much about choices between cpu fans for silence - they will all be drowned out by the rest of your system, particularly the 6800 ultra. The quietest air cooling you can get for that is an NV 5 and that will still be a lot noiser then either cpu fan.
Putting dampening on the fans and hd's will make a difference.
The other thing is sticking good quality sound damping material to the inside of the case (I use acousticpack standard), this really takes the edge of the noise.
I don't know if your case has a door in front of the cdrom drive, etc, but putting sound damping material on the inside of that and shutting it makes quite a bit difference as well.