You are providing mixed questions which are hard to follow. The only thing I got out of them is a computer with at least two NIC or using a consumer grade router. The consumer grade Linksys WRT54G router can use dd-wrt. You can use an old computer and add two NIC to get what you want. Another possibility is buying a motherboard like following and add more NIC by using a module card.
Jetway J7F2 VIA C7
3x LAN Module or 3x GigaLAN Module
If you want a motherboard that is more up to date and graphics card is easy to install, the following could be used.
Jetway JNC62K AM2/AM2+ GeForce 8200 Integrated Graphics
More can be found at.
A microATX or ATX motherboard can be used too.
Just do not spend money by selecting the fastest processor. VIA C7 1.2 GHz is over kill for a router+firewall+file server. Also an AMD Sempron X2 is again too much.