If you install Snow Leopard on top of Leopard it won't affect your iLife applications or any of your data. It only upgrades the operating system's core applications and services.
You should always backup your data, as good practice, just in case something goes wrong. Imagine if your computer loses power in the middle of the install or you get a power surge, or a huge magnetic storm passes through your area

So yes, go ahead and update to Snow Leopard, but backup first just in case!