Well, I think I would start with the eagle file for the ethernet card and add components to that.
Add the ATMega328 only requires a few components - uC, xtal, 2x 22pf caps, 4x 100nF caps, 10k resistor, small diode.
ICSP header pins for downloading bootloader if needed, USB/Serial header pins for down loading sketches, or adding USB connectivity out to a PC while debugging.
Don't know what PIR you have, I don't they use very many components.
How are you powering? Do you need a 3.3V or 5V regulator? Or will you bring one or both in directly?