Eheim 1048, for the same reasons
decodeddiesel mentioned.
This is my YY cube case with a watercooled Athlon XP:
(the Eheim 1048 is resting on a piece of foam)
The pump is real quiet, but it vibrates some so decoupling it from the case is an issue you have to deal with if you want a real quiet system, as with any pump I guess.
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Pump:Eheim 1048,
Radiator:LPD radiator 100x150x20mm,
CPU block:DD Maze-2,
GPU block: DD GPU block,
Chipset block:Zalman NB32
AMD XP 1500+, EpoX 8K7A, PoV GF3Ti200
Pic of my YY-cube