You could take a look at the new Soekris 6501-70 [http://soekris.com/products/net6501.html]. It should be able to handle 100Mbit and be an integrated solution that prevents you from needing to use consumer PC parts. We’ve used Soekris for years and they’re reliable and stable. You could also add in a gigabit dual-Port Intel server adapter for the expansion slot and you’d end up with 6 gigabit ports, dual core CPU, and 2 GB RAM. Which is a lot of power and options for bonding and interconnection.
You could also look at Soekris’s 5501-70, since they can push around in the range of 50-75Mbits depending on how you’ve got it set up. This will decrease the more packet manipulation you use. (We currently get around 30Mbps sustained out of it but there’s a lot of manipulation happening)
As a last note, look into hacom.net [http://www.hacom.net/]. They primarily try to bundle with pfSense with it but I’ve seen Zeroshell [and Vyatta] run quite well on the gear. They give you quite a few different options, the VIAs are great and so are the Atom platforms. I tend to stay away from the ones with Realtek NICs because I’ve rarely seen where those NICs didn’t act like garbage…