DG45FC is a mini-ITX desktop motherboard and it should fit in an ITX chassis however there is an another issue I'd like to highlight here. You see, you are trying to build a server providing multiple services such as AD, DHCP, Exchange and File Server even though it's for a limited number of users where I have a couple of reservations.
First, this motherboard is a desktop board that supports following Microsoft OS:
Windows XP Pro 64 Bit
Windows Vista 32/64Bit (WLK 1.1)
Windows Vista 32-bit/64-bit (WLK 1.2
I have not been able to find any official document by either Intel or Microsoft stating that Microsoft SBS (Small Business Server) 2003 qualifies for this motherboard since it's pretty much understandable for SBS being a server OS should be installed on a server board. My point being, be it Microsoft or any flavour of Linux, Unix and even Novell, server OS is supposed to be installed on a server class motherboard which obvioulsy would have a compatible processor to balance the load on a server due to multiple services and numerous users requesting those services.
Secondly, I and a thousands of people out there very rarely install a server class OS on a desktop motherboard which might just be used for testing purposes or evidently a transition however a server deployed in a production environment, I'd never suggest that you go for a desktop motherboard. Server class motherboards and processors are designed to bear the load and they simply do not max out easily which a desktop board with a compatible processor is very likely to.
Installing SBS 2003 on this board should work however I still do not encourage you to do that for post installation problems will definitely surprise and exhaust you i.e. Drivers, compatibility issues of SBS 2003 with reservations on a desktop board, etc.
Hope you get my point, kindly let us know what you have to say on this and let us know in case you require further help.
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