First, i 'd like to clear out the terminology. I want my ADSL router to let the internet connection "pass" to a pc, that will work as a router, firewall, etc; the router will only work as "switch" and i will "bridge" the router and the PC.
I have also heard, that i need a client for this- a Windows one. Is there a Linux client?
Thanks

a client? what do you mean? this kind of setup should be 100% architecture agnostic. it's nothign more than ethernet on bits of wires.

You have some options:

If you connect just one machine to your router, and disable all firewall services of your router (i.e. open all ports and everything else to that machine), then the router is effectively pass-thru and you can use the machine for whatever you want.

Connect just one machine to your router and if your router supports it, set up that machine as the DMZ host (you configure that on your router). Then you can do whatever.

The problem with both of those cases (well maybe not with DMZ, I honestly don't know much about it), is that your router is still a router and not a switch so there will still be some NAT going on and the one machine you have connected to the router will still have a "LAN" IP.

If your ADSL modem has ethernet out, you can connect it directly to your machine. In that case you will need to set up PPPoE or whatever you need to use on your machine (the router normally let's you set this up, but if you remove the router you need to do that from the machine itself). That may be the "client" stuff that you were hearing about. I don't know how to set that up on Linux (but I know you definitely can), but you can probably Google for it if you know your connection method. In this way the computer would entirely take the place of the router's firewall and port mapper, and you can get a regular router/switch/whatever and put it on the local side of your machine. Of course, this doesn't work if your ADSL router is also the modem or if your DSL modem is putting out raw DSL line data (which looks like an Ethernet jack but the data is not the same... it's more like a beefy telephone line).

You'll need two ethernet cards in your computer, of course; one for the ADSL side and one for the LAN side.

In any case, hardware firewalls/routers are usually better (both in efficiency and security) than using a computer and their software counterparts, unless you have a particularly buggy router.

Hope that gets you started,
Jason