I want to replace my iMac's internal HD (320Gb, to be replaced with 1Tb). Am I correct in thinking that, once the new disk in place, I can just start up from my SL DVD, format the new drive and restore my TM backup (which means no reinstalling SL) ?
Yes. Or you get an enclosure for 3.5" S-ATA HDDs and connect it to your Mac and use CarbonCopyCloner (up to version 3.4.4 free) or SuperDuper! to clone the internal HDD to the external HDD and then make the swap and use the older HDD as backup or extra storage.
Instead of a case (unless you want to put your old HDD in there later) maybe try a HDD dock. The drive sits in the top, put in your new HDD, use CCC or other software to clone the internal. Switch drives and it should boot with nothing more to do.