Wouldn't you need a motherboard with two ethernet jacks for that?
As others have pointed out a Switch/Router will work or if you have a spare slot you can add a PCI NIC for $20.00 and you will get top speed. I love my NAS I can sync my laptop at work in less than 15 min, I am syncing a 500 GIG drive (about 22 GIG of data) which would take 6+hours through a USB 2 port.
This stuff has come a longg way in the past 2 years and set-up properly it is almost effortless. Although I still have a few users who "Fook" it up regularly...lol