Hi Guys!,,, nifty situation here,,
as you dont outline if the primary PC is XP,, i will assume that it could be,, Vince is correct on all counts for the network problem with the frequent power outs,, syncs, IP's/ hanging IP's could cause major network connection issues, basically NONE
If you make the primary PC an XP pro machine and multihome it, you will be able to have one pc pulling one IP from the ISP, connect this to one of the NICS,,,from the other NIC, feed it to an Uplink on a HUB,, make sure it is a 10/100 for performance issues,, then,, this is where the XP internet sharing kicks in,, plug the other machines into the Hub...
On the primary PC,, XP there is a Shared networking wizard that will make a diskette to run in each NON XP machine to add it to the network,, the Prime PC will give DHCP addresses automatically in the 192.168.x.x range,,
Now as for access, you are able to choose if the Prime PC is controlling the Internet access OR if users that logon Wake the Prime PC and the Gateway is activated at THEIR LOGON/powerOn..
If you choose that the attached machines can only have internet access if PRime PC is on,, then that PC dictates Internet Access,,,
other than that there is a programme called WatchDog that will kick users off at specified times configured by you,,,
Just some ideas,,Hope it helps,,