Using the intel d201gly as a m0n0wall router should work very well. It needs an extra nic card though. I just sent this to the m0n0wall list:
You all might want to consider the new intel d201gly mini-itx. Its less than $100, takes cheap ram, and has a pci slot for an extra nic. Combine that with a mini-box m300 ($60), a stick of ram ($30), a compact flash card ($25), a power supply ($50), and the extra nic ($25) and you're in great shape for a couple hundred bucks. If energy consumption is a big deal, then this probably isn't the right machine for you, as it uses about 25 watts of power.