Though I've never used it, Slackware is highly recommended as a stable distro by many people and its also popular in the Linux community in general.
I use Fedora Core 1 and I have no problems with it.
It is really a matter of choice. As long as you use a stable kernel, then the distro shouldn't really matter. KDE and Gnome have a tendency of slowing down the system and sometimes apps *do* crash in KDE/Gnome. You can use a lightweight Window Manager if you really want a fast and stable experience or stick with pure-text more command line.
Difference between Linux and FreeBSD? I'm not a technical person, but I think that others might help you out with that.