I current have two Mini-ITX based M9000 systems and I am a great fan of these, however be under no doubt that these are not powerfull in any sence of the word.. The 933 mhz Via C3 which powers the M9000 generally benchmarks out about as well as a Intel Celeron 600.
If you are just going to be using it as a small office machine, media centre, or a firewall then the ITX format is oustanding, however a lot of people consider these for LAN party boxes or a general PC and in my experience they have a lot of problems it being used for this. The mini-itx format is a low heat, low power system and fitting such things as certain Geforce PCI graphic cards (no AGP) can put large demands on the tiny PSU and produce low voltages for the mobo.
If you require power look towards the SFF (small form factor) mobo's such as the Flex-ATX or Micro-ATX
What are you going to use the system for ??