20 GB of free disk space on one partition is hardly enough space for video imports. 1 hour of DV takes up 12 GB on its own. I'd actually consider:
1. Cloning your hard disk at least twice*:
http://www.macmaps.com/backup.html
2. Repartitioning it so you have about 25 GB set aside for temporary scratch space on your video imports.
3. Or getting a larger internal hard disk. Your machine can take up to 160 GB internal 2.5" Parallel ATA (ATA/IDE, and NOT SATA), and repeating step 2.
Hard disk space typically needs to be contiguous for smooth video imports.
And while Mac OS X does its own defragging on 20 MB and smaller files, larger files can get really space conscious.
Downgrading via a clone, erase and install, and careful recovery of your data can work too, but there may be compatibility issues with Leopard data in Tiger. An archive and install of Tiger over Leopard can yield many more compatibility issues. I don't think Leopard's entirely the problem, but rather how you use your disk space.
- * Links to my pages may give me compensation.