use any of the up to date distro's redhat6.2 or above, and Mandrak 7 from my experience work fine. The earlier kernals dont support(or crash) certain network modules needed for dsl.
make sure you get an external DSL modem.. I dont believe that there are any internal DSL modem cards that are supported in linux. This may have changes in the last couple months so look into it. But i would still recommend an external dsl modem.
For DSL that uses pppoe(point ot point over ethernet) look into software called Roaring Penguin. It supports pppoe connections, and its free. Next do some research on ipchains, this is the software you'll use to setup your firewall rules(what can come in, what can be forwared..etc...).
As for using you firewall as a multi purpose box..hmmm Technicaly you shouldn't. the firewall is meant to be a barrier between the outside world and your network, running something on the only visable part of you network kinda defeats the purpose. On a practical level.. I use mine as a web server to check and develop my web pages(minus sensitve info) before uploads.. But thats the extent of it. I actually store all my html stuff on an internal box also.. so just incase my linux box gets hacked or something, I can wipe the box clean and start fresh with no loss to myself, except the time to reconfigure the box.
