Well, you can always go with pre-made watercooling rigs if you feel like it. Personally, I would have gone down this route if I didn't have the dying urge to mod my case. Of course, there is also the possibility of adding water cooling to your existing case.
Here are some things I did to quiet my rig. The sound is barely audible, and my system temps usually lie within 10-15 degrees of ambient, and my cpu temps usually 15F of case temps.
-Antec TruePower 430 Watt: (Uses 2x92mm fans at low speed. Also has thermal control to spin fans faster/slower when needed)
-SLK-800 heatsink with 80mm YS-Tech variable speed fan: I usually have the fan running at ~ 2000rpms: inaudible and it gets the job done because of the killer heatsink ($50, but worth every penny for the peace and quiet)
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ZM80-HP To replace the fan/heatsink on my ti4600. No fan = silence!!
-4 80mm stealth fans on baybus usually running ~1200rpm, also inaudible.
Also relevant is that I'm using an ASUS A7V333, which uses a large heatsink on the northbridge, eliminating the need for a fan. The case I use is an Antec 1030/1040b.
Sys specs:
Athlon 2200+
Gainward Ti4600
512MB Corsair XMS PC2700
My system is fairly powerful, although I'm sure you can get a quiet system with more powerful specs. In fact, the temperatures on the 2400+ and 2600+ are actually lower (on average) than the 2200+ due to the increase in die size and thus, surface area for heat dissipation.
I'm not familar with P4 systems, maybe someone out there can comment on their solutions.