If you are new to linux, and have never setup this type of system before, I'd recommend either Mandriva, or Suse, as both of these are geared for ease of use. Mandriva would be my first pick, as it has a fairly comprehensive set of configuration tools that will handle all of the hard stuff (but you can also get at the config files easily enough). Ideally, if you have an old spare drive (at least 10G), use it as your OS drive, then the other drives can be dedicated to storage. Since the drives aren't completely uniform in speed, and you want to be able to install new drives in the future, I'd suggest configuring them as logical volumes using LVM. This way, adding another drive is as simple as plugging it in, marking the partition as an lvm drive, and adding it to your existing lvm pool. As with any type of raid system, though, it can become a problem if one drive fails, but that is usualy rare these days.
I have a server running this way (Dual PIII Xeon 550, 128M memory, 1 10G boot drive, 1 40G LVM drive, and 1 80G LVM drive), and it has been running flawlessly (up 401 days). I am getting ready to add another 100G drive to it soon, but I use it primarily for ISO's (Mandriva, Suse, CentOS, Fedora so far), and of course, MP3/Ogg files (~60G). The biggest problem I have ever encountered with this situation is space in the case (it only has 2 HD bays, and 1 3.5" floppy bay). I ended up ducttaping the boot drive to the top of the CD-Rom, with a piece of cardboard in between.