As ibnabouna implies it depends how much money you want to spend and I’d also add what needs you have for your system. If you’re into top level gaming hardware then it’s going to be expensive to get things quiet. e.g. water cooling.
However if your needs are more moderate then buying a fanless motherboard and video card will reduce a lot of the noise (small fans are the worst). I also found one of the quiet power supplies made a big difference.
Here are a couple of links that you’ll find useful:
http://quietpc.com/
http://www.hushtechnologies.net/
I’ve bought several items from quite pc and have been happy with their service. Hush has always been a little bit more than I wanted to spend.