A much better step would be to
1: Clone the existing 10.5 to a Disk Utility HFS+ journaled formatted external drive using the free Carbon Copy Cloner
2: Hold option and boot from the clone, check it out. Disconnect.
3: Upgrade 10.5 to 10.6 on the orignal drive using the 10.6 upgrade disk, see how everything works (if not you can hold option and boot from the 10.5 clone and even reverse clone if necessary)
4: If the 10.6 upgrade went well, then clone that to the new larger drive (formatted HFS+ journaled first of course)
The reason for this is you avoid ANY issue with using Setup or Migration Assistant, which isn't exaclty perfect or even close.
You also get a external clone as a backup.
If your thinking, "I'm going to install 10.6 so it's fresh" all that goes to HELLO when you use Setup or Migration Assistant, you migrate all the crap from 10.5
If you really want a fresh install of 10.6, you need to do 10.5 install first so you get your free iLife, then upgrade to 10.6, then install programs from original sources, finally just copy the files from the 10.5 drive.
If your doing what your doing to optimize/defrag the drive, cloning back and forth does that as CCC copies folders/files alphabetically via the root directory.