It depends what you mean by silent. By silent do you mean 1) quieter than your reserator? 2) Quieter than Ambient room noise level (which varies quite a lot) or 3) absolutely no moving / noise generating parts
If 1) maybe try modifying your reserator? Is it possible to rubber mount the whole unit - or better still just the pump withing the reservoir? You might be able to stop a lot of the resonance.
If 2) You're going to have a hard time making a P4 system that quiet. Not only do you have to dissapate a large amount of heat from the cpu and then exhaust it from the case, you also have extra heat being generated in the PSU which you need to lose. Of course, if you were using an A64 this would be much less of a problem. To give you an idea - my overclocked 3000+ runs fanless using a sythe ninja heatsink. If you undervolt the chips to 1 or 1.1 volts you'll produce less than half the heat - probably under 25W full load. Most a64s will still run 2ghz+ even at 1v. It makes it easy to run them passively with carely any airflow in the case.
You haven't mentioned your other components though - Hard drive and Gfx card. Are you happy with the noise level of these? Hard drives are extremely tricky to get quiet enough once you are trying to achieve this level of quietness. Its a struggle with even the quietest 3.5" HD - the samsung spinpoint. Once you make one component quieter - you are just going to hear the next in line! People (including myself) go to the extremes of even using laptop HDs, suspended and even encased in all manner of materials in order to achieve a truly sub ambient noise level.
I think what i'm trying to say here is it's easier to start with quiet components in the first place
3) The nirvana of the silent pc builder! I have plans for a machine, completely fanless, using an undervolted A64, 120W power brick and picopsu, and passive cooled gfx ... but you'll always get into trouble with data storage. Ideally you'd have enough local solid state memory to run the operating system etc and set up some network attached storage for the rest..... finally a truly silent pc ..... but of course solid state memory is still prohibitively expensive for this kind of task so back to "nearly silent" with a quiet laptop drive, damped and enclosed
I think I've gone off track a little here as I suspect you are only really looking for answers to no.1. If thats the case, your best bet is to search the net and see what mods people have done to make their reserator quieter. If you want to go that step further, step 2 is achieveable but it's a slippery and expensive slope
Step 3 is for dreamers
Good luck
Marc