Fedora Core 6 - upgrade or re-install
Re-install would be better option. The latest fedora is fedora 11 and lot of things would have changed from Fedora 6 to Fedora 11 including
- Location of some config files
- Format of config files
- Default set of tools which get installed with Fedora
You should definitely install it from scratch.
Even if you perform a step-by-step update, you most likely get problems with even between the "small" update steps.
Because of that I would even think about a reinstall if it were just an upgrade from Fedora 10 to Fedora 11, but going from 6 to 11, it's a definite yes. You'll save time.
I disagree with DR in that it's very easy to perform an upgrade from one Fedora version to the next with recent versions - see this link.
Going from 6 to 11 is a different matter, though; I agree with the other posts that the changes are significant enough that you'd be much better off doing a reinstall.