If you already have 10.6.7 why are you upgrading to Snow Leopard? It's already installed.
Updating does not erase the drive, so none of your files are affected in the process. But there are no guarantees that all will go well, so it's wise to make a backup of what your have. Preferably, a bootable backup by cloning with Disk Utility to an external drive or another internal drive.
Clone using Restore Option of Disk Utility
1. Open Disk Utility in the Utilities folder.
2. Select the destination volume from the left side list.
3. Click on the Restore tab in the DU main window.
4. Select the destination volume from the left side list and drag
it to the Destination entry field.
5. Select the source volume from the left side list and drag it to
the Source entry field.
6. Double-check you got it right, then click on the Restore button.
Destination means the internal or external backup drive. Source means the internal startup drive.
The backup drive will be erased in the process, so don't use one with files you want on it.