[bobkoure]

IMHO 28dB should be pretty straightforward.

Have you already bought the case? The 180/182 is very nice - for air cooling. Once you get into water cooling the separate airflow path design of the 180/182 is (IMHO) more hindrance than help. I'd look at some of the larger Antec server cases - particularly if you're going to hog out a big-ass hole in the lower front for a radiator/intake. Sorry I can't recommend a particular one - their models have changed since my last build.

Whichever case you get, mass-load all the panels. I've had very good luck with Elemental Designs eDead V.1 Sound Deadening - which is a butyl core with adhesive on one side and heavy foil on the other. About a dollar a square foot at ED. You can layer it, (the adhesive sticks to the foil just fine) and you probably should figure on two layers. It's easy to cut with a box knife or scissors.
Once you've got that installed (to deal with pump vibration), then add a layer of acoustic foam. ED's got some, but I haven't used it (have a lot of dynamat stuff in my garage, waiting to be used up).
The loading is to deal with pump noise (well, pump vibration that can cause panels to vibrate and those cause the noise. The foam is to kill the high-pitched noise from your drives.

The best way to figure what (other than side/top/bottom panels - which you should just mass-load) is to hook up your pump so it can run (maybe just a loop with a reservoir) and let it lie on some sheet metal in your case - which will let you see how your vibration-killing is working before you install motherboard, etc.

Once you've got that about as quiet as it's going to be, get some "gel stuff" from Petra's tech shop (amazing stuff - about four dollars for a 3" by 3" pad) and use that to isolate your pump.

About the only other thing you can do to kill pump noise is to use soft silicone tubing to/from the pump (the tubing's the remaining vibration path). The downside is that this stuff lets coolant evaporate slowly, so you'll need a reservoir.

The key to reducing fan noise is to have as large a radiator as you can fit (preferably using case-intake air). Thin is also good as it reduces the pressure required to flow air and loses very little in cooling capacity. There's lots of discussion about this here on PC - and the Thermochill PA160 came out of that discussion - so at least consider that rad.
I've had good luck with Nexus 120mm fans, both for case exhaust and for pulling through a radiator (if you are using a heater core you will need to "stack" them in order to get sufficient pressure while still slowed way down). They don't "tick" when PWMed.

There are quiet power supplies out there. I've had really good luck with Seasonic.

I think that's about it. My current PC isn't as powerful as what you're looking at (OC'ed AMD X2) and it's not completely inaudible - but I have to stick my head under my desk to hear it (mostly a whisper of disk noise). BTW, I've got the exhaust fans speed-controlled on case air temp and the rad fans speed controlled on cpu-block temp. I expected it to get louder when the room temp's up and I've got the CPU at 100% (mpeg encoding, mostly) - and it does get a bit louder, but I still have to move under the desk to hear it (a little whoosh added to the disk whisper).

Hope this helps - use/ignore at your pleasure...