See the news forum on this site for some links to free Linux and Knoppix "books", the posts should be near the top of the most recent articles. There is plenty of other information on the Internet as well, a little searching with google will turn up more than you could ever read, but the books are a good place to start.

I don't know about your modem. If it is a USB modem there may be some issues, and I don't believe in or support USB modems in any way. That is not to say that there might not be a Debian driver for it, but I would avoid it. If it's an ethernet modem then it will work fine with almost any common 10/100 ethernet NIC in your system (and some 10/100/1000 NICS) and Linux. I would strongly suggest using a router, it's absure to run a high speed connection without the firewall protection one offers (the router is positioned between the modem and your computer in the ethernet network). A software firewall is not a replacement for a hardware firewall in a router.

Debian would install fine in 4 gigs and leave you plenty of room for additional packages. Of course, it's always easy to fill that space with more applications and more data. Debian could be installed in 1 gig, but it would not be a comfortable fit if you also tried to install the KDE Graphical User Interface. KDE is somewhat larger than others GUI's and will eat a lot of that space. You could certainly install Debian without a GUI (just a command line or "shell: textual interface) or even with a smaller GUI, there are many available for Linux such as Gnome that need less disk and memory space than KDE. 1 gig will be tight though, assuming that the purpose of this is to actually install some programs and have some data rather than just to install and have a Linux operating system, but it can be done.