Noctua make some very nice (but expensive) fans. The Akasa Apache is a very good fan also and competitively priced. High airflow but extremely quiet. I use both in my rig. The Apaches are on the CPU cooler (for PWM control) and case fans are Noctua. Only downside of Noctua is that they are 3 pin fans vs 4 pin PWM like the Apache, but since you are going to use them for the case then not an issue. There are many other alternatives and everyone has their favourite manufacturer I guess, but if noise is an issue it can definately be worth spending a little extra on the high quality models. If you're on a budget, the best cheap and cheerful fans I've used so far have been Akasa's regular range.
