You absolutely, positively do not want to do this. You may think you do, but you don't.
The system may not need every little piece of software that it ships with, but you have no easy way to figure out which it does need and which it doesn't. In addition, there's simply zero reason to remove Apple's built-in software. Instead, both MacPorts and Fink will automatically place all of their software in a completely distinct, non-Apple directory (/sw for Fink and /opt/local for MacPorts). Upon installation, they adjust your
$PATH variable so that when you go the command-line, you automatically find their version of software first rather than Apple's (if there are two versions of the same thing).
You can then easily and cleanly use either Fink or MacPorts (or both) and never touch the Apple stuff. If at a later point, you decide that you want to remove all of Fink or MacPorts, you can easily do that by blowing away the custom directory with a two-second
rm -rf. (You can always cleanly uninstall individual packages using Fink or MacPorts's built-in tools.)
Just for completeness, there are two other interesting tools for getting extra *nix software on a Mac: Rudix and Homebrew. Rudix is interesting to me because it's highly modular: you can install any of its applications one at a time as a simple dmg. On the other hand, it offers fewer packages than either MacPorts or Fink. Homebrew is very new and very interesting, but it requires more from the user in terms of initial command-line knowledge. Since it's new, it also has fewer packages than MacPorts or Fink.