Huff said:
Archive & Install. It shouldn't touch anything under your home folder, so you'll be left with a fresh Tiger installation with all of your personal stuff intact.
That said, if you want to be ultra paranoid and ultra safe, you might consider waiting until you finish that project before upgrading. OS upgrades, no matter how good, are a big deal and especially with the first release of a new major version, there are a lot of unknowns. No major company would (or should) do such an upgrade in the middle of a critical project, at least until they've had time to test it offline and/or get feedback from other users. If you feel your data is that critical, you might consider this.
At the same time, I know how tempting it is to jump in and upgrade.
My copy should be arriving today, but I've been planning to hold off and read reports about any issues before taking the plunge myself. Like you, I have gigs and gigs of important data, and I don't want to lose it because of some obscure bug that never came up in Apple's testing. It's extremely unlikely but possible. For example some people with certain FireWire drives had them wiped out with Panther, and the iTunes 2 installer had a bug that could delete an entire disk partition if you had your disks setup in a particular way. Caution doesn't always hurt. We'll see if I can even last the weekend before breaking down and installing, though.
I'll be using Archive & Install whenever that happens.