Quietpc.com. That should be your first step to getting a quieter machine. You might have the same fans as what they sell, but check their power supplies and quiet drive systems. I sprung for their for their graphics card fanless heat sink. If it moves in a PC, it makes noise. (Quiet PC usually has DB ratings on their products, too!)
Secondly- Start with your preferred sound card, what I've found time and time again that the MOTU and Digi stuff always plays well with Intel chipsets and processors. (Not for sure about RME and others. . . ) I'm not knocking the AMD world at all, I've just found that almost every install/help me I'm going to shoot myself call seems to have the AMD word in it. Make sure before you pull the trigger and get your mobo that the interface manufacturer supports it!
(It sucks because I'm an AMD believer for about everything else . . .)