Higher capacity SD (8gb+) cards seem to have an issue with the kernel right now. The RPi wiki has a list of tested cards. I don't think powering off your auxiliary carpcs in the back like that is a good idea, though I'm not really sure what your alternatives are.
I guess I am going to have to get it, and play with them. What size SD card should I get for the units? Also, can I set the cards as read only? I ask because I just want these units to shut off. They will be controlled by the main PC via a relay. Is that possible?