yup its preeetty silent.. good enuff for recording. Sensitive condenser mics might still pick up a slight hum tho..
Besides the casing, u gotta think about the cpu fan as well as the graphics card fan cause these are main noise generators also. Try to get a graphics card which is fanless. For cpu fan, I'm using a zalman 7000alcu heatsink, the one that looks like a flower.. quiet and efficient.
I also put in an extra large low rpm cooling fan in front of my harddisk drives. The key is to choose large, low rpm fans cause these are much quieter than small high rpm fans.. Antec makes these as well.
Motherboards from Asus also have got a fan speed throttling feature which throttles down the cpu fan when heat is at acceptable levels.
best of luck in your search..
cheers