Well if you have the budget you can get a nice noiseless pc case like the zalman ones..
http://www.zalman.co.kr/ http://www.tomshardware.com/2004/01/...ing/index.html
expensive but nice..
Otherwise some simple things like choosing a motherboard without a chipset fan, and vga card also without a fan, and low noise power supplies and cpu fans make a big difference..