I recommend using the Disk Utility Restore function. You can put the new drive in the external case first (or test it in the case, then install in the Mac), then boot to your Leopard or machine Restore disk, and run Disk Utility from there.
I've used this method for years with a perfect track record. Whether you put the new drive in the external case to clone, or install it in the Mac, then clone, doesn't really matter. I like to do the latter, though I put new drive in the ext. case, initialize it and run some basic tests to make sure it's working (initialize it using GUID partition table, single partition, and Mac OS Extended (journaled) format). Then I install it in the MacBook/MacBook Pro and put the old drive in the ext. case.
Here are the steps I use to clone the old drive:
Boot from your install DVD (or the Mac's restore disk), run Disk Utility, and verify the source disk with First Aid.
If you hadn't done so already, format the new drive with a single partition using GUID map and format as Mac OS Extended (journaled).
Next, use Disk Utility Restore to clone the original to the new drive. Drag the old volume to the Source and the new to Destination. Check the "erase destination" to enable block copy. Double-check source & destination. Check it again. Disk Utility will ask if you are sure you want to erase the destination. Check your source & destination again.
Wait a while (it could easily take an hour or two, depending on how much data is being moved).
If you already have the new drive installed in the Mac, then just restart. The startup disk should remain set to the internal, but you can verify and restart from the Startup Disk app. Otherwise, now's the time to swap drives. Once that's done, you can restart per above. You should now be booted on your new drive.
If you have a Boot Camp Partition and Windows installed, it can be cloned to the new drive too. I've had great results using
WinClone:
With the old drive mounted, use WinClone to create an image file of the old BC Partition.
Next, use the Apple Boot Camp Assistant to create a new BC partition on the new drive. It needs to be the same size or larger than the old, but larger only works if you had formatted the BC partition as NTFS. Actually, it will work with FAT32, but you won't have access to the additional space.
Finally, use WinClone to restore the image file to the new drive's new BC partition. Windows should boot right up. I've done this method several times over the last month and it's worked fine. Also, if you had created a new, larger partition AND it was NTFS format, WinClone will expand the old partition to the new BC partition size.
The other option, as suggested, is to do a clean install and just use the Migration Assistant to transfer your apps and data. You can put the new drive in the external case or do it as I mention above (new drive installed in the Mac). Doesn't matter, really. But, if you aren't having any trouble with your install, a clone is much easier, IMO.
Enjoy the new BIG drive!