It's really up to how savy you are. I have installed Mac OS X on a few machines and it's really quite easy. All I'd suggest is give yourself about a week before hand and start backing things up. Don't rush into reloading or you might miss something. Confirm you have all your documents and such backed up, and try to get any informations regarding installed programs you have loaded/registered/purchased. Once everything is backed up, start the upgrade process.
Now, a side note, I'm very, very, VERY much against upgrades. Unless it's an upgrade from a clean install, problems almost always persist. The drive is also more cluttered etc etc... I'd strongly advise doing a fresh install if at all possible. This is the reason I suggest backing up everything... If not to do a clean install, just in case something does go wrong and you're forced to roll back or do a clean install.