Also, there are some nice fanless Mini-ITX boards out there with dual gigabit, which is all you need (plus a case for a board and a switch, of course).
M350 enclosure is popular and cheap.
It goes without saying such box will smoke anything you can buy of-the-shelf. Hell, even the anemic ALIX Geode gave a nice boost to my home network over the crappy Linksys I had previously (things are now snappy and 100% stable).
Soekris now has new models with Intel Atoms, top quality stuff as always (old ones are rock solid too, if a bit dated). I went with ALIX instead as I had a local distributor so it was quite a bit cheaper. Plenty fast for now, too.
http://www.newegg.com/Product/Product.aspx?Item=N82E16856205...