I used G4U before to backup from one disk to another.
This is a very nice program, but it has some inconvenients.
The main problem I had was that it does not handle well
the disk geometry differences. To make it simple
if your disk has not the exact same number of cylinders,
the same block size and the same number of block by cylinders :
You may have problem. The backup is not lost,
but your disk will be badly formatted.
You will lose the extra space if you have more blocks by cylinders
on the new disk. My new disk was seen has having 40GB like
the old one... but it really had 120GB. So I ended up connecting the
old disk with a cable adapter that does USB to IDE .
Reformatting the new disk, booting with a live distro (knoppix)
and copying everything on the new drive.
Don't forguet that Ubuntu use the UUID of the disk. You have to
change that in /etc/fstab if you want your new partitions to be
seen. The command to list the UUID of a disk is "blkid".
If you have space on the new disk. I would reserve a "backup"
partition and from a live CD, cd to the root of each of your
partition and take a backup with tar.
The command to do it would look like :
"tar -cvvjpf /media/backup/disk_num.tar.bz2 . "
(the dot after bz2 is important,
and you need to be cd in the root of your disk).
If something is wrong, then you can alway boot with the live cd
and bring back your data with :
"tar -xjpf /media/backup/disk_num.tar.bz2 -C /media/yourdisk"
I have no experience with clonezilla.
I hope this info will help.