Today I finally assembled my HTPC. There were some stock issues with the case I ordered, so after a week I ordered another. Although the motherboard is passively cooled I decided to enable one of the two case fans. Unfortunately the fans are very noisy when directly connected to the motherboard, so I placed a simple Zalman fancontroller between the fan and motherboard.
After building the HTPC I first installed Ubuntu 10.10 using a USB stick. Then I used this guide to install XBMC.