120mm case fans, you really shouldn't need more than two of them, and organize the cables and things in your case such that there is good airflow between the front and rear fans. Unfortunately I'm not sure how well simply replacing the fans will work with your case if it's designed for 80mm fans since I have a case that originally had 120mm fans.
You can also try replacing or slowing down any CPU heatsink, video card, motherboard chipset, etc. fans that may be loud. The most annoying, if not the loudest, fan in my main system is actually the one on the chipset, because it makes an incredibly annoying high-frequency sound when it's running full tilt. I fixed it by installing a
Zalman fan controller to reduce the speed about half, and the temperature is none the worse. I also have for the CPU
another fine Zalman product which is big and has a big controllable fan which is the best you can look for when you want quiet. It's running about 25 degrees F over room temperature. (granted Athlon; I can't guarantee any level of performance on a P4.
)
Another computer of mine, that I don't use and thus haven't bothered to fix this, has a CPU heatsink fan that is incredibly loud like there is a jet engine running out behind the building; the moral of the story is replacing even one part can make a big difference. (it has only three fans total: CPU, GeForce 2, power supply, and it's much louder than my main with seven: two 120mm case fans; CPU heatsink; a stock X850XTPE cooler, which is surprisingly quiet because the fan never runs in "dustbuster mode"; the previously annoying chipset fan; and two in the power supply)
The bottom line is, you certainly don't
need water cooling to have a quiet high performance system. It's probably easier if you originally buy components with noise in mind, but I'm sure there's still a lot you can do like this to assuage the problem.
PS this site is really helpful:
http://www.silentpcreview.com/