If you want low power, do not go P4. Ugh.
Go with a micro ATX Ivy Bridge or Haswell board, something inexpensive like a cheap H65/67 board or an H77 or something. Then slap the cheapest Celeron processor on there you can, maybe 2GB of Low voltage RAM and be done with it. Oh, get an Intel dual port gigabit ethernet adapter. They are typically pretty cheap, especially if you buy one of the ones that was out a of a proliant or something off eBay. I think I've seen them for $30-40.
That should cover it if you really need dual ports.
Or if a single port is fine, maybe just get the cheapest Intel NUC you can. $150 + a SODIMM + 8GB USB thumb drive buys you all the hardware you need for a fairly robust router (so long as you only need a single port) and its probably the lowest power consumption you can possibly get, especially if you eye up that new Intel Bay Trail based NUC, which retails around $150. Bonus points for included a wifi card in it already, so you could run it as a router AND a wireless access point.
I mean, if you want to go super lower power and cheap, A Raspberry Pi should handle things fine, so long as you don't need to route a huge amount of traffic or super high bandwidth (considering the 10/100 port on it).