Half an inch should be fine for the RJ45 and the power plug. Actually, gimme a minute, and I'll go drill a half inch hole in a block of wood and test it out..


EDIT:
Okay, just tried it through a 3.5" thick chunk of wood. A 1/2" hole will NOT pass an RJ-45 connector at all. A 9/16" hole will pass one, but it is a close enough fit to make it very difficult.

No problem at all with just the ethernet cabling and then a DC-power cord from a WRT54GL.

Cheers