I use a Supermicro X7SPA-HF ITX board, it has an Atom D510 and 2x Intel Gigabit NICs (on PCIe, be aware that a conventional PCI bus is too slow for 2x gigabit).
In a test setup I could do torrents at 400 mbit. I don't recall it maxing out the CPU, but I didn't bother looking into it further as I only have a 120 mbit connection anyway. 400 mbit smells like a maxed out harddrive of one of the test computers.
It has been running 24/7 since Feb 2011, no problems with it at all. Only reason for reboot is a software update which has been 500+ days ago, don't tell anybody.