When you unplug it the bios clock will still be run on a battery for an undetermined amount of time..
somewhere between 7 and 10 years depending on battery quality and type.
Your computer probably doesn't have a remote wakecall capability.
Any ATX based computer has wake-on-lan as long as there is enough power on the standby (500mA-1A@5v). you just need to turn it on
Well what was said above and a side notw, on some/all motherboards if not unluged the PSU supplys small amounds of power to the system to run the clock etc. Thats why you ulways switch off/unplug the PSU directly before messing with components or you could get fryage.
ATX isn't 'hard switched' anymore (that means that there isn't a 220v cable going from the PSU to the power switch) so it needs power on the board to detect the power switch contact impulse (which in emergencies can be done with screwdrivers, paperclips and any other thing that is small enough to fit between the pins and is conductive)