Thought i would add that i have built my system now anyway, one of my main concerns was power, as I generally don't see the point in wasting in it for wasting sake, just so that i can say well if i ever need to push 500Mbps of traffic i can, because i wont ever have to do that.
I built up the following system;
Intel D945GCLF using a 1.6GHz atom processor
2GB DDR 2 533
On board 10/100 WAN
Intel Pro GT 1000 LAN
Small Cube case
Now it uses 35w of power and sits nicely in the bottom shelf of my computer desk, rather than the probable 200W my main rig was using to power all the fans hardware.