I can install dd-wrt/openwrt on a linksys/asus router and get away with it...
and then host the webserver on another server... hey but I am just a little bit more eager home user with some linux hobbies... I really dont want to host a number of servers in my home (I already have a big desktop, a HTPC and another machine for webserving is no good idea... also I dont want outside traffic entering inside my LAN.
well, in this case, why is a third NIC (DMZ/other uses) a requirement? does not make much sense.
as far as I understand, you're looking for three different things: a kind of a gateway appliance, a webserver open for requests from WAN, and a fileserver serving the LAN.
I would not mix those all up on a single host. Your gateway is the most potentially exposed host on your network, so you _really_ don't want your data (filserver services) on this host. if your webserver is open to requests from the internet, then I would not consider putting it on the same host. even better -- keep it in a separated network, and do your ip filtering between this network and LAN wisely. this is, basically, what DMZ is good for in such a setup.
you could also consider running gateway services off one host, fileserver services off another one, and maybe running the webserver services off a virtual host running on the same physical host as your fileserver services. this would potentially give you some more control about what's happening than mixing it all up on the same host, but also would raise the hardware requirements a bit.
if the webserver services don't have to be open to the public, but instead only to you or to some controllable amount of clients 'outside', you could consider using some kind of a VPN setup, possibly even serving the web-stuff off your fileserver.
by the way, what kind of web services are you going to run?
another point to consider are the quite different requirements for those services -- it won't be easy to combine them all in one piece of hardware while keeping the price low.
Also, _do_ avoid the mentioned VIA EPIA SN10000EG board. it's heavily overpriced, has a crappy GbE NIC (RX locks up after some time if you'd increase MTU above 1500, at least with the via_velocity driver in a bunch of different kernels in the 2.6.27-2.6.30 range), SATA controller has mediocre performance and a severe bug in it's AHCI implementation, rendering _theoretically_ possible NCQ useless. the single nice feature to consider is crypto-acceleration stuff buit into the CPU, which may come in handy if you are running high-throughput VPN connections.