I am running SME 7.3 on Micro ATX motherboards, but I use AMD products. I usually use Gigabyte MB's, and have been such as GA-M61VME-S2 With a Sempron processor, and 512mb to 1GB ram. Add a couple of hard drives (PATA or SATA), a cd-rom if you want (or just temporaraly to load SME) and a second nic for server-gateway. Works great for small to medium offices.
I Like the micro-atx because I don't need to add a video adapter. I just turn down the amount of shared memory to allow most possible for the system.
Bob