Quiet PC

For some good discussions on quiet computing, go to www.silentpcreview.com

Water cooling is really air cooling; you still have a fan, it is just located in a different place.
If the cpu cooler or the liquid radiator have the same 120mm fan, expect their noise to be similar.
Nothing wrong with your Xigmatek. Yes, there are better coolers at twice the price, but the improvement will be marginal, and probably not worth it.

In general, it is the fan speed that makes a PC noisy.
In particular, the fan speed of a strong graphics card such as the 6970. Not much you can do about that.

A good cooling case will have several large slow turning 120mm or 140mm fans that will be quiet.
Cases with windows are noisier than cases with insulated panels.

I like the direct exhaust coolers for graphics cards. They get heat out of the case, keeping the cpu cooler.
Some aftermarket coolers do a good jpb on an open test bed, but may not do as well in a closed case.

I can think of a few suggestions:
1) Get some sound deadening material for the case panels. Yes, it will cover up your view of the innards.

2) Graphics cards run hot, but they are built to do so. There are programs that will control the fan speed of your 6970. Try reducing the fan speed, accepting that the card will run hotter.

3) Look into replacing some fans with ones that have a better efficiency to noise ratio. I would not hope for too much here.

4) If you game with a headset, you will not hear the noise of the PC.

5) Turn down the AC in your room. Cooler intake air to the PC will lower the need for fast spinning fans.

The effort and expertise involved in a installing a full water cooled system is beyond most home builders. You have to be very careful: water and electricity are a dangerous mix.

I had the same goal: build a quiet & powerful gaming machine. After much searching I found the Antec H20 620 to be the best mix of good CPU cooling and quiet. It has the advantage of liquid cooling, but as a closed loop none of the hassle of exposing the mobo/case to liquid. There are other closed loop liquid coolers, but Antec seems to be the best brand (with their 920 model tops for cooling).

I kept the 200 mm Corsair case fan in front because it is very quiet. I modded out my other case fans (a Corsair 120mm rear & Corsair 200 mm top) with three Xigmatech 120 mm Crystal White fans. These fans are rated to be below 20 dB.

Some case fans are rated at 35 dB +. Stay away from them. Replace all fans with ones that are rated below 20 dB.

I lined my case doors with noise dampening foam. This reduces reverb and echo of the fan noise, but is the last step, and is the least effective of noise reduction.

I disagree with geofelt on the shrouded GPU coolers. Pushing hot air through a small aperture is like pissing through a pin hole: you get lots of warm backflow. In my tests I found that the open design GPU coolers are both quieter and more efficient. They do the job of getting the heat off the GPU and let your case do its job: exhaust the heat. Plus the open exhaust coolers can run a lot quieter at lower fan speeds.

Incorrect. There are many options for new folks that are very easy to install. Even a full-custom loop is simple to install and maintain if you do your homework first and don't cut corners. If the OP wants to look into watercooling, give the links in my signature a read-through (very thoroughly). Depending on budget, you can put together a decent loop for low cost. There are other things to consider...so do some research and check into it.

Some time ago, in the pentium days, I had a problem with cpu temperatures.

In desperation, I replaced the stock graphics cooler with a direct exhaust cooler, Arctic cooling, as I remember.
The results were amazing. BOTH the cpu and gpu temperatures dropped by 5c.
A cpu or gpu cooler can only approach temperatures of the ambient air in the case.
By getting that hot air out of the case as fast as possible, you give the cooler colder air to work with.

If you have a case with superb airflow and cooling, then a more efficient cooler makes more sense.

Some of the current graphics cards seem to have smaller outlet grilles. EVGA will sell low restriction grilles for the GTX580 type cards. I don't understand why they are not standard. Every little bit helps.

Good case airflow trumps any CPU/GPU cooler upgrade. Without good airflow, you can't expect a cooler to work as designed.

If you pull the side of your case and blow a house/desk fan (on high) into your case, then benchmark and see lower temps...you have an airflow problem. If your temps remain the same, you can then consider a better cooler.

Larger fans, i.e. 140 and 200mm vs. 120mm, mean more CFM at a lower RPM. Lower RPM equals less sound.