Linux firewall

[DingleBerry]

I've got a epia I've been using as a mail/subversion server for a while. Since it needs linux reloading on it anyway i thought I might as well set it up as a firewall too. What kind of tools are there about for this kinda thing? I'd prefer to use ubuntu server as a base as I know my way around it. Are there many advantages over just using what ever the router does (i thnk its a netgear of some sort)?

My router has got an internal hub, I guess I'll have to bypass that and use an external one?